You can try the disk repair option in Disk Utilities, but if that doesn't do it, then there's really probably nothing you're going to be able to do short of formatting the memory card in the camera. I suppose you could try a few utilities specifically for FAT32, which is likely what the card is using, but I wouldn't hold out a lot of hope. Among the many failings of the FAT filesystem is that it is not overly fault tolerant. It's pretty easy for it to become corrupted and there being little to no recourse.