I've got the same thing going on.
The good news is that I believe I know what has CAUSED it -- using "Clear Data" for the "Media Storage" application.
I have a lot of image files (maps) on the tablet and SD card. When I (re-)insert the SD card, teh Media Storage application runs, and fill all the available memory, so that nothing can run. If I "Clear Data", the tablet works again. But the tally of "Used Space" goes up...
The bad news is that I have no idea how to fix it )-:
Robert.