Hmmm... sounds like something is very wrong somewhere in the filesystem / partition area.
I'd try the following:
* Ensure you have enough space somewhere else to store a copy of the files. Doesn't matter where, if it's in the PC, the Mac, another USB disk, or a collection of all three.
* Use the Acer to copy the files to somewhere else. I don't have an Acer A500 tablet myself, so I don't know if it can do this - but if it can, that's probably the safest bet to recover the data. If you can't get your hands on another large USB drive, using a smaller drive (even a thumbstick) to copy files to the PC or Mac in stages will work, even though it's fairly laborious.
* Find a Linux box somewhere and try reading the drive with that. Maybe you've got a friend with a linux box who can help. If not, download a knoppix or similar live-CD, boot the PC from it, and try with that. A linux box should have much the same drivers as the Android tablet (Android is based on Linux), but without any of the tablet restrictions, so that's probably the best way of getting it readable on a computer.
* Try doing a full disk error check via the windows box. If you can't do it via tools / check now, then do it from the command line. The command you need is 'chkdsk /f e:' (where e: is the drive letter assigned to the disk). If you haven't already got the data back off the disk using one of the previous two methods, this might return everything to normal, or it might give you a bunch of files with generic names and no directory structure (i.e. all the TV is there, but you won't know what episode is what without watching them and renaming them), or if you are unlucky it might give you a working filesystem with nothing on it at all.
* If the worst happens, you didn't manage to back anything up, and you end up with an empty filesystem, try an undelete program/tool. FAT is a very basic filesystem, and you'll likely recover a LOT of information this way.