Recovery from failed fastboot flash (return to 'DROIDBOOT PROVISION OS' from 'white USB symbol' problem)
I have made the dangerous attempt of trying to flash a 'fastboot droidboot.img' from a similar specs ZenFone 2, for which people already figured out how to unlock the bootloader without waiting for Asus.
That work was reported here.
and I've seen that the official firmware 22.214.171.124 for the ZenFone 2 ZE551ML (downloaded from here ) contains indeed a 'droidboot.img' file which the ZenPad S8.0 CA allows to flash, without complaints about signatures.
However, upon rebooting via 'reboot bootloader', the 'Droidboot provision PS' no longer shows up. The screen shows a white USB symbol, no more, and is no longer visible from a PC through 'adb devices' or 'fastboot devices'. The rest still works; but have we lost fastboot forever?
NO: if the device was rooted before (strongly advised...), a trick allows to flash without passing from 'fastboot' at all.
1) Get the official full firmware archive from Asus ( file UL-P01M-JP-126.96.36.199-user.zip ), and copy it on the device.
$ adb shell
3) became root:
4) move that archive on a 'safe' place, i.e. /data
# mv /storage/emulated/0/UL-P01M-JP-188.8.131.52-user.zip /data
5) magic trick: create a recovery command via:
# cat > /cache/recovery/command <PRESS ENTER, CONTINUE WRITING UNDERNEATH>
6) restart and go through that script:
# reboot recovery
The device will reboot, and start a reinstallation which will, in particular, restore the Droidboot system and get rid of the white USB symbol.
Fastboot is back with us! ;-)
$ adb reboot bootloader