I'm starting to think, since it sometimes can read your files and sometimes can't, that it might be a problem with the actual hardware that reads the data off the drive. In which case you'd need to get it repaired and I hope it's under warranty.
But first check the disk for errors like zach said. Seriously.
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"What if you just eat vegetarians?"-neonreaper
"^ You are what you eat. ^_~"-Koiji