If you have a new Samsung Android smartphone which was released with or updated to Android 9, you may notice that the usual key combination to enter Download mode (Volume down + Power button) does not offer the ability to unlock the bootloader.
On previous models, you would be asked if you want to enter Download (Flashing) mode (short press volume up), or Unlock bootloader mode (long press volume up), or to exit (press volume down). Now, you would just be entered into Flashing mode.
To unlock the bootloader, you have to enter another Download mode which does contain the prompt to unlock. To do this the fastest and most painless way, you need to follow these instructions:
- Enable Developer mode in the About screen (tap Build number until it shows “You are a developer”)
- In the Developer mode enable OEM unlocking
- Turn off the phone from the OS
- Press and hold Volume up and Volume down, but not Power!
- Plug in a USB cable connected to a PC
- The phone will enter the Download mode and prompt you to unlock
- Long press Volume up until the screen changes
- Press Volume up to unlock
- The phone will unlock and reboot, and all data will be wiped!
The procedure to relock the bootloader is the same, instead of Unlock it will just say Lock, when the bootloader is already unlocked.
NOTE: Newer Samsung devices do not actually unlock the bootloader fully, because they have a new VaultKeeper service. To actually unlock the bootloader and avoid the “RMM STATE: Prenormal” error and having to wait 7 days, follow instructions in this post.