a hard drive will reallocate a sector, when a sector goes bad, or a bad spot on your drive. some drives come with bad sectors, some acquire them over time. either way it means your hard drive isnt perfect. i would backup any important data and replace the drive.
Thanks for the reply.
All I have on there are back-ups of back-ups. So no big deal. I'll just pull it out of my case and stick with my other drives.