[HTC U12+ Guides, News, & Discussion] 5m4r7ph0n36uru:
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Hello everybody,

first I have to inform you that this thread is to be regarded as highly experimental. I would like to warn all users, and advise against using this guide, who are not yet fully familiar with Treble A/B devices, and do not know how to debug problems!

If you continue, you are doing so on your own responsibility. I cannot accept any liability and/or responsibility for any problems that might occur during the process, such as bricks and other incidents. If you proceed from here on, you agree to personally take full responsibility for everything you do!​

Prerequisites

You are familiar with the Universal HTC RUU/ROM Decryption Tool and know how to use it.

You have a current RUU.zip for your device downloaded onto your PC (ATTENTION software version has to be the same as on your currently activ full stock slot!)

Your bootloader has to be Unlocked/S-ON

You don't panic when something goes wrong.

Guide

First: Start from scratch! What I'm saying is that the first thing you have to do is RUUing back to Stock.

Second: Never, I repeat, never use a PIN/password for the boot! Of course you can use a PIN, a password, or a screen lock pattern, but never for the boot process as long as you want to use a dual boot!

Okay, let's get started:

Use the Universal HTC RUU/ROM Decryption Tool to extract the system.img and the boot.img using the -sf command.

Copy system.img and boot.img to your fastboot/adb folder on your PC (NOT system_other.img, NOT boot_signed.img).

Rename the RUU to 2Q55IMG.zip, and copy it to the root directory of your extSD

Flash your RUU to restore a full stock system to your primary slot, slot A.

Power off your device

Reboot to download mode

Hold the power button pressed to power up your U12+.

Keep the Power button pressed and wait for the very first vibration.

In this very moment you got to release the Power button, press the Volume Up button immediately and hold it until the bootloader shows up.

If you did everything correctly, you'll now see the bootloader mode.

Navigate with the Volume Down key, until "DOWNLOAD MODE" is shown on the top of the screen.

Press Power once to enter it.

Flash your stock /boot and /system images to inactive second slot

flash boot.img to current inactive slot, using

fastboot flash boot_b boot.img

flash system.img to current inactive slot, using

fastboot flash system_b system.img

(this one takes a while to start, and will be flashed in three steps, so give it some time! It'll show an error first, so let's hang it on that very error for some seconds - see post #2 for further details.)

Reboot your device to active slot A, and set everything up

Activate Developer Options and check USB Debugging

Reboot to bootloader using adb reboot bootloader

Change active slot to slot B unsing

fastboot --set-active=b

Reboot to now active slot B once, and let it settle

Now you can start to modify slot B according to your wishes. So I run slot A as full stock system, while I have installed ViperU12+ in my slot B CleanSlate kernel, Magisk and Magisk, having a custom slot and a stock slot.

If an OTA update is offered, you only have to switch the active slot back to slot A, use the

fastboot --set-active=a

, reboot the device and install the update.

DISCLAIMER: the modified slot will be completely overwritten during this update process, so you should only install the OTA update when a new RUU is available, if you want to keep the "dual boot" option. If an OTA arrives and there is already a RUU for it, you have to install it after changing the slots as just described, and carry out the steps 1 - 12 for the now inactive slot again to realize a second modified slot.

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XDA:DevDB Information

HTC U12+ (IME) - „Dual-Boot” Slots A & B

Contributors

[MENTION=5832952]5m4r7ph0n36uru[/MENTION],

Created 2018-09-08

Last Updated 2017-10-17