It could be a sign that your hard drive is going bad. Small sectors of
the disks will become unreadable and the capacity of the drive will
decrease as those sectors fail. You can continue to use it as long as
you have all your data backed up in case it does decide to die all of
the sudden. I had this happen on a really old system. The drive started
having the occasional bad sectors, then it was happening every boot-up,
then a week or two later it refused to boot.
