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I'm sorry for asking a questions that's been asked before, but I have read conflicting information, so I want to be absolutely sure about this before I proceed.

I am aiming to ROOT my Pixel 4a (5G) with Android 12.

Will flashing a boot image modified by Magisk wipe all my data? (I have installed Magisk Canary because of Android 12, correct?)

I have already unlocked my bootloader, and of course that wiped my data. I don't want to go through that again. I don't want to have to reconfigure my setup again. (Some apps didn't automatically install for some reason. A few more of them lost their configurations. And system settings...)

Anyway, can someone please give me an authoritative answer?

Many Thanks, ---Mark

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    Unlocking the botloader is the only function that unavoidable causes a data loss. However there are still multiple things that can go wrong. In those cases your data will not be erased automatically but it may happen that your device ends up in a boot loop and nothing you do fixes it except may be erasing all data. If you modify the device there is always a chance to lose your data, it may be not very high but who cares about chances if you are in the end affected?
    – Robert
    Jan 14 at 16:01
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I'd conclude it as no. It should not cause data wipe on your bootloader unlocked Pixel device.

Most well known triggers of data wipe are:

  1. bootloader unlock and relock,

  2. manually triggered factory reset,

  3. automatically triggered factory reset

The third one is something harder to talk about. In your case your device is a Pixel device running closest to what vanilla Android has to offer. There is no known (not to me) feature wherein a vanilla Android automatically triggers a factory reset if it detects boot image tampering on a bootloader unlocked device. Furthermore, while I am not certain I don't think Android OS provides a way to device administrator or owner apps to ascertain if the boot image has been tampered with since last boot. As such while a device administrator app can issue a data wipe, it has no reason to. I do suggest that you go into Security settings of your device and see if there is a device administrator app that is enabled as device admin and also has "Data erase / Factory reset" permission. If it does have that you would want to revoke device admin permission from this app before continuing with the process of boot image modification.

As for non-Pixel device owners, all I can say is that you do some related search on your nearest search engine to see if something comes up. A very simple search query such as "flashed modified boot image on my XYZ device Data wiped factory reset" might bring up some useful results if your stock ROM is configured by your vendor to automatically erase user data on detection of boot image modification even on a bootloader unlocked device. Having such configuration enforced for a bootloader unlocked device does not make sense though, and I have not really come across any such case either.

Over and all, you have no reason to worry about at this point. Your device's bootloader is already unlocked so (1) would no longer be triggered. You are not going to trigger (2) because you actively don't want to. And there is no known feature in vanilla Android that would trigger (3), so go ahead, modify or hack the boot image as you see fit.

A strong suggestion: when dealing with hacks and modifications in Android it is best to take a reliable backup of your current state of device and store it in PC. When I say backup I'm not talking about what Android natively offers. That thing is very inconvenient and unreliable. Instead, use a custom recovery such as TWRP which takes backup in a manner you would expect from a backup tool. You give the command to backup and a custom recovery obeys that religiously with superuser privileges. When you ask it to restore a backup it does that too very religiously.

A less powerful alternative to a custom recovery was sugested in the comments. Izzy mentioned that the app SeedVault works better than Android's native backup and restore solution. Here's the relevant excerpt from his comment:

...luckily, Seedvault (the replacement integrated with e.g. LineageOS since Android 10) is way better [than Android's builtin backup and restore solution]. I had to perform a clean flash to upgrade to Android 11, and almost everything came along. Wherever an app had opted out from backup, data was gone – but at least the APK was carried over, regardless of its source: F-Droid, XPosed repo, sideloads – that was a positive surprise.

Go ahead with that solution that suits your requirements of data backup and restore, be it native solution, custom recovery solution, SeedVault, or something else you would come across.

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No it won't. Flashing bootloader image wipes bootloader only.

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