Create a Full System Partition Backup on Android with ADB and Fastboot

1. Verify ADB connection

1. Connect the phone to the PC via USB.
2. Open a terminal (Windows Command Prompt, macOS Terminal, or Linux shell) and run:

   adb devices

   You should see a line like 0123456789ABCDEF device. If it shows unauthorized, unlock the phone and tap “Allow USB debugging”.
3. If no device appears, reinstall the USB drivers (Windows) or ensure adb is in your PATH.

2. Unlock the bootloader (if not already unlocked)

Most manufacturers require an explicit unlock command. The process varies:

• Pixel / Nexus: adb reboot bootloader then fastboot flashing unlock. Confirm on the device with the volume keys.
• Samsung: Samsung devices typically do not expose fastboot; you’ll need to flash a custom recovery (e.g., TWRP) to obtain fastboot‑like access. This tutorial assumes a fastboot‑compatible device.
• Other OEMs (OnePlus, Xiaomi, Motorola): The command is usually fastboot oem unlock or fastboot flashing unlock. Check the manufacturer’s developer site for the exact phrase.

Warning: Unlocking erases /data on most phones. Ensure your personal backup is complete before proceeding.

3. Reboot into fastboot mode

From the ADB shell, issue:

adb reboot bootloader

The screen will show a fastboot prompt (e.g., “Fastboot mode”). If your device uses a different key combo (Power + Volume‑Down), you can also power off and hold those keys manually.

4. Identify the system partition name

Run the following command while in fastboot mode:

fastboot getvar all

Look for a line like system: /dev/block/bootdevice/by-name/system. The exact name (usually system) is what you’ll use in the next step.

5. Pull the system image to your PC

Use the fastboot flash command with the --download flag (available in newer fastboot versions) or the fastboot oem variant. The most universal method is to use fastboot pull:

fastboot pull /dev/block/bootdevice/by-name/system system.img

If your fastboot version does not support pull, you can use the following workaround:

1. Boot the phone into a temporary recovery that supports adb sideload (e.g., TWRP). This does not require flashing the recovery permanently.
2. From the recovery, run adb shell dd if=/dev/block/bootdevice/by-name/system of=/sdcard/system.img.
3. Then pull the file: adb pull /sdcard/system.img ./system.img.

Both methods create a system.img file (typically 300‑600 MB) on your computer.

6. Verify the backup integrity

After the transfer finishes, run a quick checksum comparison:

sha256sum system.img

Note the hash value. Later, after a restore, you can recompute the hash to ensure the image wasn’t corrupted.

7. (Optional) Compress the image for storage

System images compress well with xz or zip:

xz -9 system.img   # results in system.img.xz

Store the compressed file on an external drive or cloud storage. Keep the original hash in a text file for future checks.

1. Re‑enter fastboot mode

adb reboot bootloader

2. Push the image back to the device

If you kept the image compressed, decompress first:

xz -d system.img.xz

Then flash:

fastboot flash system system.img

Some devices require the fastboot flash command to be prefixed with fastboot --disable-verity --disable-verification flash system system.img to bypass Android Verified Boot (AVB). Use this only if the device refuses to flash otherwise.

3. Re‑lock the bootloader (optional but recommended)

fastboot flashing lock

Locking restores the device’s security posture. After locking, the device will reboot and may perform a factory‑reset – be prepared.

4. Reboot the phone

fastboot reboot

The phone should boot into the exact system state captured in system.img. If you see boot loops, you likely flashed an image that doesn’t match the device’s firmware version. In that case, repeat the backup on a matching firmware.