[Android Q&A, Help & Troubleshooting] jkmartindale:
My dad recently bought an HP 10 tablet from my uncle and asked me to root it...which was a mistake. I had a successful root with the Android-based version of King Root, cleaned it up with SuperSU Me, and thought everything was fine until I rebooted...and the tablet got stuck on the boot animation. Booting with the volume down button down resulted in just the HP logo, without the boot animation. Booting with the volume up button down booted into recovery. When I boot into recovery I'm greeted by this lovely message at the bottom:

E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/log
E:Can't open /cache/recovery/log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_log
E:Can't open /cache/recovery/last_log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_install
E:Can't open /cache/recovery/last_install
E:failed to mount /cache (Invalid argument)

I have tried the following things from the recovery menu:

wipe data/factory reset:

-- Wiping data...
Wiping /data...
Formatting /data...
E:failed to mount /cache (Invalid argument)
Formatting /cache...
Data wipe complete.

wipe cache partition

-- Wiping cache..
E:failed to mount /cache (Invalid argument)
Formatting /cache...
Cache wipe complete.

apply update from internal storage

E:failed to mount /data (Invalid argument)
E:error opening /data/media/0: No such file or directory
Install from internal storage complete.

apply update from external storage

E:failed to mount /cache (Invalid argument)
E:failed to mount /extsd (Invalid argument)

(Tablet was booted with microSDXC card inside)

apply update from cache

E:failed to mount /cache (Invalid argument)

apply update from ADB

adb was...weird. I verified the connection through adb devices, but most commands, including adb reboot bootloader, resulted in error: closed.

Windows side:

C:\Users\jkmar>adb devices
List of devices attached
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
CNU4389ZNS sideload
C:\Users\jkmar\Desktop>adb reboot recovery
error: closed
C:\Users\jkmar\Desktop>adb sideload UPDATE.ZIP
loading: 'UPDATE.ZIP'
sending: 'UPDATE.ZIP' 100%

Tablet side:

Now send the package you want to apply to the device with "adb sideload <filename>"
...
E:failed to mount /cache (Invalid argument)
E:failed to set up expected mounts for install; aborting
Installation aborted.
E:Can't mount /cache/recovery/log
E:Can't open /cache/recovery/log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_log
E:Can't open /cache/recovery/last_log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_install
E:Can't open /cache/recovery/last_install
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/log
E:Can't open /cache/recovery/log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_log
E:Can't open /cache/recovery/last_log
E:failed to mount /cache (Invalid argument)
E:Can't mount /cache/recovery/last_install
E:Can't open /cache/recovery/last_install
E:failed to mount /cache (Invalid argument)

UPDATE.ZIP came from HP's download page. I just don't know what to do at this point.