[dontbeevil]

I hope there's somebody here who can help me, as the only threads which I've managed to dredge up anywhere were posted by those who didn't understand that their SD card inevitably has up to a couple of GB consumed by the system formatting software. The problem can be seen in this screenshot:
The system storage display shows two "Internal Storage" groups, but the second one is the on-board "sdcard". It's supposed to contain 11.44 GB, which is a lot more than the "Available" and all the other displayed accounts under this group add up to - in fact, there's nearly 10 GB which have vanished into a black hole somewhere! The "Available" space was down to zero, before I removed some applications and moved data to my extended SD card (displayed as "extsd" in my file browser, but listed as the second "Internal Storage" group under Android Settings->Storage). After moving or removing all that was possible, this would render my tablet basically useless (Matricom Gtab if that matters) without my SD card, and still I can't have more than a few basic apps without running out of space because even when I move my apps to my "SD card", it arbitrarily moves them to my wonky internal SD, no option to move them to my external card. That isn't quite so big a problem as the fact that many apps save data to the internal card without giving me the choice (not just Play), and that's already filled up to the point that I'm getting installation errors from Play.

Please help if you can, even if it's to say you are sure it's too weird to fix and the device needs to be returned to the manufacturer. I have already tried the factory reset, to no avail.