Strange Hard Drive Issue

My roommates PS3 is having this strange problem. It seems to think that it's hard drive is full. When I plug in ACII it says it needs anoth 600MB of space to create data. We went through all the things on the hard drive and the numbers don't add up right. He says there was some issue with it where some data got corrupted and it has had this problem ever since. Does anybody know what we can do to fix this? The only thing I can think of is formatting it is kind of annoying.