Well, it's hard to guarantee an up-to-date database without rescanning on boot; the user could have taken it out and changed it in another system. I could be wrong, but I don't think FAT stored last mount time for comparison; if other systems can be trusted to change a time-stamp on modify, then a fast scan could be made of all timestamps, comparing each directory to a stored copy made at shutdown. (More generally, this could be applied at every mount/unmount.)