When manually updating my Nexus 9 (Wifi) to Android 6.0, I forgot to flash the cache.img and vendor.img images. Now my Nexus 9 is stuck at the Google boot screen (just says Google, no animation). How do I get the Nexus 9 back to the state where I can re-flash images manually again (i.e., "flashboot flash...")?
The solution was simply booting in fastboot mode (I already was there). Apparently, booting in fastboot → plugging in the USB cable → waiting a couple of minutes for the device to be detected by ADB (adb devices) works best (otherwise it won't list the device). I was then able to install Android 6.0 successfully via fastboot commands (USB debugging and OEM unlocking were previously enabled and the bootloader unlocked).
fastboot flash bootloader bootloader-flounder-3.48.0.0135.img fastboot reboot-bootloader fastboot flash recovery recovery.img fastboot flash boot boot.img mfastboot flash system system.img fastboot flash vendor vendor.img fastboot flash cache cache.img fastboot reboot
mfastboot was used instead of fastboot to flash the system.img file because the latter gave an "could not allocate -1995847632" error message (it can't handle big image files).
I will re-lock the bootloader later.
