i think that happened to my usb drive the other month, i couldn't right to it or anything it appeard full there were jibberish files all over it. i tried unlocker no success, so i ran a file recovery app on it and reformatted it i had no other choice
and some of my data was dammaged it sucked but it whent away after i re-formatted it.
i think it happened to mine when i pulled it out while the mac i was using was reading from it..