Reinstalling Gracenote had no effect, so it seemed to be a cache issue. But I found a fix: I formatted the drive and gave it a new volume label, then put my files back on. Then the system displayed all the info correctly. It seems that changing the volume label makes the system think it's a different drive and forces it to build up a new cache/database from scratch.
I still have no idea what made it fail in the first place, but at least I have a fix.
I still have no idea what made it fail in the first place, but at least I have a fix.