Usb drive - Not formated

I have a creative muvo slim mp3 player 512mb, evrey once in a while when I was runing from it aplecations (Pstart, Thunderbird, Portable Gaim) I pluged it in to a new computer and the computer would claim it was unformated, and after that any other computer (diferent OS {all windows}) would claim the same. Formating the drive fixed the problem (and erased all info). The problem does not acoure when i dont run aplecations from the drive.
Also the Mp3 player still played the music (mp3) on the drive, when it was "unformated".
Any Ideas? Any one else with this sort of problem?
Askrra