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TWRP says no OS is installed, but I just flashed it. Can anybody help?

Hi,
Its been a while since I been attentive to my Pixel. I was a lucky one who managed to get a Verizon 7.1.2 with an unlocked bootloader. I unlocked the bootloader, flashed a ROM, and rooted and haven't had a single major problem since.
I was running Resurrection Remix from about March and I yesterday I wanted to try to use Android Pay, so I followed a tutorial I found in another thread here to install Magisk, and when it was all said and done I ended up in a boot loop where the phone would reset as soon as it reaches the Google logo, ramdump, and eventually boot back into TWRP.
In the attempts to delete out all kinds of caches and dalviks and factory resets and whatever else, nothing worked.
So in the end I decided to just flash a new ROM. Flashed the latest Pure Nexus, reflashed TWRP, flashed the vendor image... went to go reboot and it gives me the WARNING! NO OS INSTALLED! error. I thought that was odd and decided to just update to the latest Resurrection Remix - same problem.
At this point I am unsure what to do next. I don't care about any user data on the phone, I just want to get it back up and running with a new ROM.
I have a few questions and thank you in advance for all help you may be able to offer:
1) Why am I getting the NO OS INSTALLED issue directly after flashing a ROM? Logs indicate no errors whatsoever. /system/ is mounted. Every time I boot into TWRP it asks me if I want to Keep System partition read only. I always tell it to allow modifications and never ask me this question again - it asks every time. Is it possible that /system/ can actually remain empty even after a fresh ROM install that gives no errors in the log?
2) I believe that I am supposed to reflash to stock image that this phone shipped with to start all over. If I reflash back to stock, will my bootloader have any chance of locking again? I don't even know what version the stock image was, and I'm further confused/concerned about the fact that it was a 7.1.2 image I shipped with - from my understanding there were security updates which make unlocking/rooting impossible. I don't understand why you have to start that far back - is it even needed? The latest Resurrection Remix ROM install instructions assumes that I am on latest "Firmware" NOF26W. This is another issue I don't understand - what is a firmware? The full factory image? In any case, NOF26W is specifically for Rogers Google Pixel devices. Since I need to be on this "firmware" before I install latest, can I just flash the NOF26W Stock image despite the fact that I am using a rooted Verizon Pixel? The NOF26W image is 1.8GB: What exactly in this ZIP am I supposed to flash?
3) There were various weird things about this phone/flashing ROMs that I seem to recall such as having to boot an older TWRP before flashing ROM and then flashing the newer TWRP - is any of this stuff still relevant?
4) I also remember that many people were fastbooting TWRP rather than flashing TWRP for some reason - are there specific builds/non-standard stuff required for any of this to work on the Pixel?
I really appreciate all and any help that anybody can provide. If there's files that you're referring to, PLEASE send links. I need to get this phone back up and running ASAP.
Thanks again!!

when you wipe system, twrp wipes current active slot but when install a rom, it gets installed in the inactive slot (which will become active upon reboot). but twrp still thinks the currently active slots system is erased and complains that no os is installed.

rohitece06 said:
when you wipe system, twrp wipes current active slot but when install a rom, it gets installed in the inactive slot (which will become active upon reboot). but twrp still thinks the currently active slots system is erased and complains that no os is installed.
Click to expand...
Click to collapse
This. I just reboot anyway after seeing that message and I've never had any issues.

Update OOS 9.5.7 to 9.5.8 - Magisk Root

Hi fellas,
I'm running OOS 9.5.7 GM21AA and rooted using Magisk.
I got a notification for the 9.5.8 system update. I would like to upgrade the OS but would still like to keep root (and don't wanna restore my phone). However, I have still backed up to Google Drive, and also using the OnePlus Switch app and copied the back up to my PC.
My question is, what's the best way to go about updating the system without having it touch my data?
I know the system update will unroot the phone automatically, which I'm guessing is going to delete all my data (because my device is rooted). So is there a way I can UNROOT separately so I don't lose my files and data, --> and then after that do the system update?
Can I download 9.5.8 from the web manually and install via TWRP for example, thus not messing with root and not losing data?
Thanks for your help in advance!

Follow this https://forum.xda-developers.com/oneplus-7-pro/how-to/successfully-upgraded-to-9-5-7-t3937414

whatthekj said:
Follow this https://forum.xda-developers.com/oneplus-7-pro/how-to/successfully-upgraded-to-9-5-7-t3937414
Click to expand...
Click to collapse
Thanks for the quick reply.
Just to make sure, following those steps I shouldn't lose my data, correct?

omar0aziz said:
Thanks for the quick reply.
Just to make sure, following those steps I shouldn't lose my data, correct?
Click to expand...
Click to collapse
Correct. I didn't lose data when I did it awhile ago but I would still make a Google back up just in case of any errors

whatthekj said:
Correct. I didn't lose data when I did it awhile ago but I would still make a Google back up just in case of any errors
Click to expand...
Click to collapse
Awesome, thanks again! :good:

See here, as the exact question has already been asked and answered: https://forum.xda-developers.com/oneplus-7-pro/help/how-properly-update-t3942735
I know you did already got an answer, but there is additional info on the other thread.
In particular, I would reiterate would I posted about data retention when updating:
Yes, it should retain your data. I just updated to 9.5.8 a few days ago, and it kept my data intact (by the process of updating in settings, don't reboot, in Magisk Manager install TWRP installer in inactive slot, etc.).
But what I always say when attempting a "dirty flash" (flashing the ROM, but retaining data) is: hope for the best, prepare for the worst. Backup anything that is important to you before any big update, just in case. What should happen, and what actually happens, are not always the same thing!

@omar0aziz, no, losing root does NOT mean you lose your data. (This is not to be confused with locking or unblocking the bootloader, which will inevitably lose your data.) If you lose root, simply flash Magisk again.
The best way to update? I recommend taking full manual control of the update process. Use TWRP to update your ROM (that includes stock OOS) when you want to, and when you need to fix stuff.
- Flash ROM
If you are on slot A and flash a ROM, it goes to slot B.
- Flash TWRP installer zip
Keeps TWRP on both slots, patches boot.img
- Reboot to TWRP, not system
- Flash ROM again
This installs the ROM to the other slot, so now both slots are updated
- Flash TWRP installer zip again
Keeps TWRP on both slots, patches boot.img
- Flash Magisk
Patches the TWRP-patched ROM, allowing it to boot.
- Wipe dalvik
You can wipe data and dalvik instead for a clean flash
- Now reboot to system
(Keep in mind that in most custom ROMs, you will need to do an initial setup of the ROM, then reboot to recovery and flash Magisk again to get root.)
Consider flashing BOTH slots (A and B) whenever you update OOS, for two reasons. First, you don't have to keep track of which slot has which version of OOS should you flash a custom ROM or kernel with a specific version of OOS required as a base. The second reason is more compelling: the way the data partition changes encryption every update has nearly obviated the reason for A/B slots, in that if newly updated B changes data encryption upon boot and then something happens, you cannot revert to A because A can't read the new data encryption. So the solution is to update both A and B at the same time so both slots use the same encryption and can both access the data partition.
Or like me, run the device decrypted (i. e. without forced encryption).
Good luck!

I Am Confused About Recoveries

Please forgive my ignorance here. It's been 4 or 5 years since I last flashed a recovery. AB wasn't a thing back then, but it sounds like it obsoleted everything I used to know. Meanwhile, it seems like everyone around here has long since digested the changes and is carrying on conversations premised on a lot of background information I don't know. In short, I'm confused, and I need help.
What I Want to Do:
Flash the microG fork of LineageOS
Flash Magisk
Flash the Asus camera app
Install the unofficial port of the GCam app
Encrypt and divorce the boot password from the lockscreen as per this post (which I can't link to our XDA rejects my post, so I'm copy-pasting instead: https COLON SLASH SLASH forum.xda-developers.com/showpost.php?p=78699812&postcount=58 )
Question 1
Which recovery should I use? It sounds like there are three and each has problems. Is the following correct?
It sounds like the LineageOS has trouble flashing things other than LineageOS -- or is it just OpenGApps that's a problem? Can it flash the microG fork of LineageOS?
It sounds like mauronofrio's unofficial TWRP works generally, but can't flash LineageOS and can't decrypt the data partition if LineageOS encrypted it.
It sounds like bigbiff's official TWRP works generally, but there seem to be differing opinions about whether it can flash LineageOS; it can decrypt a data partition encrypted by LineageOS, but not if a password is set; and its permanent installation feature is currently buggy and might break the boot partition.
Question 2
Some posts suggest using different recoveries to flash different things. Is that advisable?
Question 3
Do I even need to install a recovery permanently? It sounds like it should be possible to boot a recovery using "fastboot boot something.img" and do everything I need to do. Is that right?
Are there any strong reasons why I should have a recovery installed permanently even if it's not necessary?
Question 4
What's the correct install order? LineageOS (microG fork), then Magisk, then Asus Camera?
Question 4b
How do I deal with the A/B thing?
Do I have to install everything to both partitions, or does the recovery install to both partitions automatically, or is stuff installed to just one partition and then somehow magically handled with the next update?
In particular, how do I get Magisk and Asus Camera to carry over to the other partition?
The instructions for the LineageOS recovery include an extra reboot right after installing LineageOS so that the remaining stuff ends up in the same partition. Does TWRP also need the same extra reboot?
Question 5
Do I understand correctly that the GCam port is just a regular apk that can be installed later through normal means -- no flashing required?
Question 6
About divorcing the boot password from the lockscreen:
Does that post apply to LineageOS?
Does the A/B thing complicate the instructions in any way? It sounds like the files to be backed up and restored are in the data partition, so I'm guessing not.
Do I need to remove the boot password to make a recovery work? It sounds like the LineageOS recovery can handle the password; the unofficial TWRP can't even handle the encryption format; and the official TWRP is currently bugged and needs the password removed. Is that right?
Question 7
So, OTA updates for Lineage OS are a thing now? Could someone explain how that works?

q1
- Just opengapps has problems but you can use lucas own opengapps build or mindthegapps. No idea about microG.
- True. There is some difference of opinion where it should be fixed (in lin or twrp).
- It played around with it a lot before giving up. In its current state i would not use it for any rom. As soon as pin/pattern/fp/stock rom/nandroid comes into play there will be problems.
q2
For convenience or if you have problems with stock lineage recovery it makes sense at the current time. Since there is currently no twrp out that fully supports lineage flashing + nandroid and encryption i would either not use twrp at all or wait for the official twrp to be fixed.
q3
This depends on your needs. Most recovery/rom combinations won't decrypt while only fastbooted. Especially when nandroid comes into play you will encounter problems.
q4
Depending on the weather, the build you use and the already installed build/addons you get different results. In general the order of those 2 should not matter but in my experience i get different/unexpected results/problems without beeing able to reliably reproduce the behavior.
q4b
I was in your shoes a year ago I'll try to keep it short. You have 2 slots. some partitions like boot exist in both slots (you will have them twice) while data only exists once. One slot is active. The phone boots from the active slot. You can manually change the active slot from fastboot and twrp so the next reboot will boot from the previously inactive slot (but you should rarely need to do that). Ah yes the most important change is there no longer is a persistent recovery partition. Recovery is embedded as a ramdisk in the boot partition.
Assuming the rom or whatever you are flashing is done right and the recovery you are using works correctly:
- flashing a rom from recovery will flash it to the inactive slot
- flashing an addon like magisk or asuscam or gapps will flash to the active slot (thats why you need to reboot after flashing a rom, so gapps will be flashed to your new rom)
- flashing a recovery installer flashes both boot partitions (at least that is the current state) and remove magisk
- flashing a recovery will install it on the active slot be it from twrp or fastboot
Due to this behavior you will have to install twrpinstaller + gapps+ magisk + asus cam every time you install a new rom (unless the rom has it already like asus cam in the next omnirom weekly) and you will have to reboot to recovery from twrp and lin recovery (active slot change to newly installed rom).
q5 yes
q6 no clue what that even does In general i had problems/errors with so many different combinations of roms/encryption/recovery and nandroid that currently i would advise to disable
pin/pattern/fp if you want to flash a rom and or take/restore a nandroid and reenable it after.
q7 It downloads the newest build and installs it in the backgroud but you still have to manage magisk, twrp etc. manually. So i prefer to just flash everythin from recovery.

A/B slot questions

I tend to mess things up when flashing new ROMs on my Oneplus6. And I usually end up using the MSMdownloadtool to 'fix my phone' again.
* Does MSMdownloadtool install OOS on both slots?
* What do you advise to do before installing a custom (v10) rom?
* I tried doing a fastboot OOS install by mauronofrio, it didnt work. I got into bootloops after running/finishing the flash all bat file. So I used MSMdownloadtool again. I wondered if the fastboot OOS installer guide by mauronofrio installs OOS on both slots??
* Does the fastboot OOS installer do the same thing as MSMdownloadtool?
* In a lot of guides for clean install custom rom I read as first step: wipe system/data. But then I wonder why do you have to install the correct OOS version first, THEN wipe it, and then install custom ROM. Does an OOS installation leave some hidden (essential) files/partitions behind or something?
* Do you guys switch slots in twrp / or do you reboot to switch slots / or do you use the fastboot --set-active a/b command?
* Why does gapps only have to be installed on 1 slot and not on both?
* Why don't custom ROMS install themselves on both slots straight away, like when you flash the twrp installer zip.. ?
Im currently running carbonROMv8 ROM, its running fine so Im not looking to change yet. But im sure im gonna have to use msmdowndtool again when I wanna try out some other ROM in the future. I dont really feel like im having any control off the flashing process. It either works or it doesnt and when it doesnt I cant explain why..

dumbl3 said:
I tend to mess things up when flashing new ROMs on my Oneplus6. And I usually end up using the MSMdownloadtool to 'fix my phone' again.
* Does MSMdownloadtool install OOS on both slots?
* What do you advise to do before installing a custom (v10) rom?
* I tried doing a fastboot OOS install by mauronofrio, it didnt work. I got into bootloops after running/finishing the flash all bat file. So I used MSMdownloadtool again. I wondered if the fastboot OOS installer guide by mauronofrio installs OOS on both slots??
* Does the fastboot OOS installer do the same thing as MSMdownloadtool?
* In a lot of guides for clean install custom rom I read as first step: wipe system/data. But then I wonder why do you have to install the correct OOS version first, THEN wipe it, and then install custom ROM. Does an OOS installation leave some hidden (essential) files/partitions behind or something?
* Do you guys switch slots in twrp / or do you reboot to switch slots / or do you use the fastboot --set-active a/b command?
* Why does gapps only have to be installed on 1 slot and not on both?
* Why don't custom ROMS install themselves on both slots straight away, like when you flash the twrp installer zip.. ?
Im currently running carbonROMv8 ROM, its running fine so Im not looking to change yet. But im sure im gonna have to use msmdowndtool again when I wanna try out some other ROM in the future. I dont really feel like im having any control off the flashing process. It either works or it doesnt and when it doesnt I cant explain why..
Click to expand...
Click to collapse
Yoo.. So Many Questions. Will try to explain in easiest way..
1. Yes. It Does Flash OOS in Both Slot via Qualcomm Download Mode which is mainly for Manufactures to restore Device..
2. Let me clear that Most people's who face issue in booting custom ROM have their data encrypted by previous or stock ROM.
Here is Link Thread to know how to decrypt your device..
https://forum.xda-developers.com/oneplus-6/how-to/tutorial-decrypt-flash-rom-pie-oreo-roms-t3838643
My own Instructions to Flash Custom ROM (Strictly for Decrypted Device):-
Boot Into Recovery
Wipe Cache, Data & System
Flash Latest Stable/Beta OOS10 & Recovery
Reboot Into Recovery(Ignore No OS Installed Warning)
Flash Latest Stable/Beta OOS10 & Recovery again
Reboot Into Recovery
Now flash Custom ROM + Recovery(If Not Included)
Reboot Into Recovery
Flash Gapps(If not included in ROM), Magisk & No-Verity zip(to keep Device dencrypted)
Reboot & Done
For Installing Update on Same Custom ROM :-
Download the Update
Reboot Into Recovery
Flash Update ZIP & Recovery
Reboot Into Recovery
Flash Gapps(If not included in ROM), Magisk & No-Verity zip(to keep Device dencrypted)
Reboot & Done
3. Fastboot Don't always work if You try to Upgrade/Downgrade Android Version as it won't flash Critical Partitions.. Meanwhile MSMDownload tool Use Download mode which is more powerful.
4. Nope. Both are Different AS both use different mode to flash ROM.
5. Clean install required while flashing new custom ROM because Not All ROM uses Same Base. Some uses Stable OOS or some uses Beta OOS... Now, your answer is in these two build Types...
In my Opinion, Custom ROM have nothing to do with your System partition but it most like to related to Vendor Partition... Both Stable & Beta OOS Have little difference in Vendor partition data..
When any dev Build Any ROM, he uses any one of those Build as base for which he tweak/fix bugs.. That's why always Use Dev specified OOS build for avoid Bootloop/bugs..
6. I prefer last option as it's more reliable..
7. It's like when you clean install any ROM.. When you install Any ZIP, It get installed in an Inactive Slot which make those ZIP data Allowed to use in Active slot..
But when you update ROM for 1st time, You install Gapps as well.. So i don't think you need to Install Gapps on 2nd Update as it already installed in both slots.
1st time when Clean installed & 2nd time when Updated 1st time...
8. It's Because how A/B Partition works... Any zip you flash get installed in inactive slot & this applies to both Stock OOS & Custom ROM...
That's why we have to flash OOS twice before flashing any custom ROM.. Which confirms you will have same Vendor partition regardless to your current slot so when you flash custom ROM it will get booted anyways even after an update..
Hope that's helpful.. Any question or help, you can DM or reply here..

Thanks for your reply. It clears a few things up I think. I got a few follow up questions.
I don't really feel comfortable walking around with a decrypted phone. I have already been pick-pocketed once and also lost two smartphones. I would say the chances are likely that also my OP6 is gonna get lost at some point. Having it decrypted would leave it very vulnerable for people trying to access my files.
* So lets say I decrypt my phone. Is it possible then to encrypt the phone again after I have flashed a new ROM (for example LineageOS 17.1)?

dumbl3 said:
Thanks for your reply. It clears a few things up I think. I got a few follow up questions.
I don't really feel comfortable walking around with a decrypted phone. I have already been pick-pocketed once and also lost two smartphones. I would say the chances are likely that also my OP6 is gonna get lost at some point. Having it decrypted would leave it very vulnerable for people trying to access my files.
* So lets say I decrypt my phone. Is it possible then to encrypt the phone again after I have flashed a new ROM (for example LineageOS 17.1)?
Click to expand...
Click to collapse
Well, In my Opinion Every Device is Vulnerable to Hackers.. Doesn't matter if it's bootloader is Locked or Unlocked...
Your device is more Vulnerable at the time you unlocked bootloader because after that you can execute advanced commands with fastboot.....
If your device have pin/pattern, Thief gone reset your device anyway to reset pass.. Eventually your data get wiped as well....
Well, you can encrypt your data but you might face issue while migrating to other ROM if you face any bug + decrypting your encrypted device again is time taking process if you have alot of data..

I mean there is difference between someone being able to reset/delete my data than someone being able to read my data. As far as I know a thief cannot just read my data after he has stolen my device which is encrypted with unlocked bootloader. Sure, he can connect it to his computer and fastboot delete my sh!t but I don't care I got backups of my most important data anyway, as long as he/she cannot just read my device data its fine by me. But how would I encrypt my device again after I installed, lets say, LineageOS 17.1?

dumbl3 said:
I mean there is difference between someone being able to reset/delete my data than someone being able to read my data. As far as I know a thief cannot just read my data after he has stolen my device which is encrypted with unlocked bootloader. Sure, he can connect it to his computer and fastboot delete my sh!t but I don't care I got backups of my most important data anyway, as long as he/she cannot just read my device data its fine by me. But how would I encrypt my device again after I installed, lets say, LineageOS 17.1?
Click to expand...
Click to collapse
you can.. But do it only if you gone use it for long term....
Not fine to waste your whole day transfer your date from mobile to PC then PC to mobile..

how to know if it's a bootloop?

hello i was upgrading from stock 9.0.7 to 9.0.9, my bootloader is unlocked and twrp is a permanent one, the phone is also rooted. i wiped system then flashed the 9.0.9 then flashed the 4.0.4 twrp, rebooted into twrp, flashed latest magisk, rebooted into twrp and then pressed reboot into system. the phone is encrypted. the booting animation has been on for 20-30 minutes at this point. is it a bootloop? why what did i do wrong? thanks

@seth wellington
Try formatting data them reboot system. That should get you booted up.
Never wipe system or vendor on these devices and always when flashing a rom flash TWRP installer zip or you won't have twrp because of where recovery is located which is same place as boot.img and it has to be installed when flashing a rom or update to a rom or it won't be present.
How I flash OOS on my once T-Mobile now converted international/global OnePlus 6t.
Boot TWRP
Flash OOS global 10.3.6
Flash TWRP installer
Flash finalize
Format data
Reboot system
How I install a rom on my converted OnePlus 6t. This installation process varies depending on whether or not whatever rom I'm installing comes with gapps and or TWRP and of course you don't want to install global OOS on another model device. Since I converted my T-Mobile OnePlus 6t using guide and msm tool I now use global or international OnePlus 6t roms, firmware... Example Pixel Experience rom installation:
Boot TWRP
Flash OOS 10.3.6 global from repo xda thread
Flash Pixel Experience rom
Flash TWRP installer zip
Flash finalize zip
Format data
Reboot system.
Start to set up device then at some point enable developer options and usb debugging and doable any lock screen and boot twrp and flash magisk for root and reboot system. That is how I install a rom that has gapps but not twrp included in rom zip. Finalize can he found with a quick search on xda. It copies firmware to other slot on your AB partition device so that you don't have to. Saves 5-6 minutes and works great.
Edit
What you could have done is just flash your updated OOS , flash TWRP installer zip, reboot recovery, flash magisk, reboot system. That's what I would have done updating OOS ..
Hope this helps.
Sent from my OnePlus6T using XDA Labs

flash713 said:
@seth wellington
Try formatting data them reboot system. That should get you booted up.
Never wipe system or vendor on these devices and always when flashing a rom flash TWRP installer zip or you won't have twrp because of where recovery is located which is same place as boot.img and it has to be installed when flashing a rom or update to a rom or it won't be present.
How I flash OOS on my once T-Mobile now converted international/global OnePlus 6t.
Boot TWRP
Flash OOS global 10.3.6
Flash TWRP installer
Flash finalize
Format data
Reboot system
How I install a rom on my converted OnePlus 6t. This installation process varies depending on whether or not whatever rom I'm installing comes with gapps and or TWRP and of course you don't want to install global OOS on another model device. Since I converted my T-Mobile OnePlus 6t using guide and msm tool I now use global or international OnePlus 6t roms, firmware... Example Pixel Experience rom installation:
Boot TWRP
Flash OOS 10.3.6 global from repo xda thread
Flash Pixel Experience rom
Flash TWRP installer zip
Flash finalize zip
Format data
Reboot system.
Start to set up device then at some point enable developer options and usb debugging and doable any lock screen and boot twrp and flash magisk for root and reboot system. That is how I install a rom that has gapps but not twrp included in rom zip. Finalize can he found with a quick search on xda. It copies firmware to other slot on your AB partition device so that you don't have to. Saves 5-6 minutes and works great.
Edit
What you could have done is just flash your updated OOS , flash TWRP installer zip, reboot recovery, flash magisk, reboot system. That's what I would have done updating OOS ..
Hope this helps.
Sent from my OnePlus6T using XDA Labs
Click to expand...
Click to collapse
Thank you very much for your reply. Unfortunately I couldn't wait for that long and I had to do something right then. After some attempts I also lost my twrp and was left without a functioning recovery, so I had to look for help and some people helped me go through unbricking through the msm tool.
The thing is i just really want to always flash clean so that the phone doesn't lag and works properly, I had no idea you shouldn't wipe system... The guide that I followed said that when updating oxygen manually you have to wipe system so I thought nothing would go wrong. I still don't know what caused a bootloop since that guide was a verified one. Thanks a lot for your help. So if I'm reading it correctly, you have to do the wipes after you've installed the system, not before?

seth wellington said:
Thank you very much for your reply. Unfortunately I couldn't wait for that long and I had to do something right then. After some attempts I also lost my twrp and was left without a functioning recovery, so I had to look for help and some people helped me go through unbricking through the msm tool.
The thing is i just really want to always flash clean so that the phone doesn't lag and works properly, I had no idea you shouldn't wipe system... The guide that I followed said that when updating oxygen manually you have to wipe system so I thought nothing would go wrong. I still don't know what caused a bootloop since that guide was a verified one. Thanks a lot for your help. So if I'm reading it correctly, you have to do the wipes after you've installed the system, not before?
Click to expand...
Click to collapse
Quit a few pretty well known developers here on xda (AOSiP op is one and there are more) say not to wipe system or vendor in their first posts of their custom rom threads. The way I describe how I install works 100% always and is the cleanest possible installation. Formatting data when installing a new rom at the end of install process does all the wiping needed to make sure nothing is left from the past and everything is clean. I used to wipe various partitions but on other devices. The install directions I use are originally from the PiXeN rom developers op. I've been flashing roms since the Nexus S days. Not saying that means anything other than I'm kinda old now. ? But that install process has never failed me once and I've flashed I'm sure hundreds of times on this device.. When I update i just flashing the updated rom over current and don't wipe anything then flashing twrp installer zip, reboot recovery flash magisk reboot system or if gapps are needed when I reboot recovery flash gapps..Unless it's a major OS update that's always worked for me.
Possibly not formatting data caused boot loop.. I know when going from OOS to AOSP rom format data is mandatory or it won't boot system.. I never flash magisk during initial flash. I always boot system first then at some point boot twrp and flash magisk. Try that next round and see if it turns out better. The whole AB partition deal made me feel like I was really old and retarded.
If I was updating OOS I would boot recovery, flash OOS, flash twrp installer zip reboot recovery, flash magisk, reboot system. Check out the finalize script xda thread. There are two here. The original person who made finalize has a thread and another user has another finalize thread. I believe in guides section of OnePlus 6t. With Android there are always more than a few ways to achieve something you know ... But what I post here is what I've been doing and I've flashed 400-500 times at least on this device.
Sent from my OnePlus6T using XDA Labs

@flash713
I totally trust you and respect anything you say and recommend me to do. Thanks a lot for all the instructions and clarification. I've been into android far less time than you — only since oneplus one. I'm extremely bad with tech but even with that phone j somehow managed to get bootloops or half-bricks ten times less than with the oneplus 6. And back then in 2015 I was way more enthusiastic: flashing twice a month loads and loads of roms. Of course it wasn't my personal achievement, I was still simply following detailed and carefully written instructions on forums on how to flash a particular rom in a particular way, but hey, at least I somehow managed it.
Now in this case I'm assuming it's the a/b partitions they've introduced that have completely changed the process and ways of flashing to the point that I stick to the guides and still get bootloops lol. That unbricking process was a huge stress for me, I don't think I'm going to touch anything in my phone for a while now. And I'm on the latest 10.3.6 oxygen so it's alright. Thank you for your help buddy

seth wellington said:
@flash713
I totally trust you and respect anything you say and recommend me to do. Thanks a lot for all the instructions and clarification. I've been into android far less time than you — only since oneplus one. I'm extremely bad with tech but even with that phone j somehow managed to get bootloops or half-bricks ten times less than with the oneplus 6. And back then in 2015 I was way more enthusiastic: flashing twice a month loads and loads of roms. Of course it wasn't my personal achievement, I was still simply following detailed and carefully written instructions on forums on how to flash a particular rom in a particular way, but hey, at least I somehow managed it.
Now in this case I'm assuming it's the a/b partitions they've introduced that have completely changed the process and ways of flashing to the point that I stick to the guides and still get bootloops lol. That unbricking process was a huge stress for me, I don't think I'm going to touch anything in my phone for a while now. And I'm on the latest 10.3.6 oxygen so it's alright. Thank you for your help buddy
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Np. My real first name is Seth. Haha. Cool name btw. I just installed OOS 10.3.7 on my 6t. I was notified of the update yesterday via oxygen updater app from play store. Good handy app to have. I would have never known 10.3.7 was out or at least not so soon if it wasn't for that app. I believe it auto notified me too. I don't remember setting it up. Android has changed soooo much you know.. I love my OnePlus 6t though. Definitely one of my most favorite devices I've ever used. What's new today is gone at some point and replaced with "better.." lol. Not always better imo anyway. Glad I could help. Feel free to message me any time. Good day to you.