Troubleshooting Android Flashing Custom ROM / Recovery

Troubleshooting

Cannot boot into TWRP without rooting

The sentence above is somewhat misleading, "without rooting" doesn't mean "no flash", I said so because I was inexperienced, so I keep trying to boot without flashing. And I've stuck in the retrying loop until I gave up several for 2 weeks of countless attempts.
I would rather name the title to "Flashing TWRP without rooting" before anyone else get stuck in the hopeless loop again.

In order to boot into custom recovery, you cannot skip flashing the custom recovery first. Lesson learned,

No One Can Boot Into Twrp Without Flashing With TWRP Recovery first!

As a Asus device user, I did trial and error this many time until I finally learnt and come to a conclusion "we can't boot into custom recovery without flashing them beforehand".

Let me summarise the general steps after I've gone through wide amount of online articles:

1. adb reboot bootloader (boot into fastboot mode)
2. fastboot devices (to make sure your device is connected)
3. fastboot flash recovery twrp.img
4. fastboot reboot (or press and hold Volume button to boot into TWRP recovery mode)

What I've learned from this:

- Flashing TWRP will still unlock the bootloader in your device

- for Asus, flashing TWRP custom recovery will also root your device (void warranty)

I haven't seen any of my trial and error that won't root my device after flashing TWRP recovery.

You can check your device info by running command below:

  fastboot oem device-info

The Root status will show as "Rooted"

Endless re-booting bootloader/bootup screen

I encounter this problem after flashed an incorrect recovery into my device.

Potential cause: Usually, you'll run into this problem after running the command below in fashboot mode (with a unsuitable recovery file):

  fastboot flash recovery example-recovery-filename.img

Solution

To resolve this, you'll have to find the correct version of recovery rom that fits your device. If you spent hours looking for one and still no luck getting one, then you may need to consider a more generally owned by the developer community or learn to compile a custom recovery by yourself. TWRP itself is a open source project, if you need something not existence

For TWRP custom recovery, please visit: https://twrp.me/Devices/

For Asus, I usually have to keep googling at their dedicated Asus Forum - Zentalk for specific/older device's discussion.

For Asus device, if you run into unrecoverable flashing issue (bricked), you can always get back the stock (official) version of ROM/firmware from Asus official support website and flash your device back into the original device firmware.

* note: avoid losing all your data, always backup your device before flashing anything

Booting into Fastboot/Recovery Mode

1) How do I get into fastboot mode?
Run the command below in order to boot into fastboot mode from "normal UI" screen (the UI show up for normal daily use made for general user.):

  adb reboot fastboot

You can boot into fastboot mode by restart/power on your phone by holding Volume Up button (for Asus Android phone).
Steps:

1. From power off mode, press and hold volume buttons
1. volume up - for fastboot mode
2. volume down - for recovery mode
2. Keep on holding the volume button after device is powered on until the expected screen mode is correctly shown.

*note: Booting into fastboot/recovery mode doesn't render your warranty void.

2) "adb: command not found", what is missing?
Which command unavailable in your system, you can't proceed any further.
To install required tools, you need run the command above using:

• (Window) adb.exe (ROM flashing kits)
• (Unix) For Unix system, 2 choices: 
• Install full official android development tools Android Studio (follow their installation tutorials), or 
• Visit Android Studio (same url as above), scroll until Command Line Tools only section and install the only the command line tools, which will install required general tools like adb (Android Debug Bridge) and fastboot tools to access to your android devices through USB cable.

Some useful articles that worth your time (for understanding about flashing custom recovery):
Discover hidden fastboot commands
Guide to customise Android device
Install TWRP recovery on Android (no root)