Solution [posted for anyone else who's brand new to the game]:
I was finally able to get my mp4 player to again read mp3 and amv files. It took quite a bit of messing around with the Partition/Encrypt/Format tools, but I finally managed to get it without cracking open my case, reloading firmware, or anything else.
First off, yes, I do have to settle for slightly less than 2gb...but I bought the thing for $20 (including shipping); having done a little research beforehand, I knew I most likely wasn't going to get 8gb (although I still hold out hope that it's there, and it's just a partition issue, and maybe once we get some better firmware, I'll have more memory, since both Windows and Linux are convinced there's a full 8gigs total there).
So, what you have to do is move the partition slider all the way to the right, run it; all the way to the left, run it, and then back to the right (but not all the way)- whatever is NOT selected as "encrypted" is your storage space for your music, videos, etc. If, after running this tool the third time, you don't have 2 drives pop up (in my case, it's F: and H

, do it again; it seems the mp4 player will only work properly if Windows reads 2 drives (one under 2gigs, the second holding the rest of the possibly-phantom space). You may have to run this tool several times to make sure you get the absolute maximum space available for your unit.
Once you've gotten that sorted out, before moving a ton of music onto the player, I would recommend just putting 1 file on. Check to make sure it doesn't corrupt, disconnect from your computer, and make sure the file is accessible. If all that checks out, you're on to the next step. If not, keep trying the above step, and testing.
After you're sure it all works fine and you've got the max amount of space available, format. Partition/Encrypt will have left both F: and H: in FAT; format f: to FAT32. Occasionally, with this final format, you may wind up with some corrupted files on your F:; if that's the case, your H: won't format at all....go back and re-P/E. If you don't have any corrupted files after this format, you may want to format H:; it doesn't seem to serve any real purpose, as it's questionable if that space even exists, but if you format H: to FAT32, it will no longer appear whenever you plug in your player, and that leads to less confusion, and less risk of corrupting your files.
Hopefully this wasn't totally confusing, and was actually helpful to someone else.