Simply having the computer on does not wear the drive at all, since the drive will just shut itself off when it's not in use. When I buy a new hard drive (or other hardware), the old one it replaces usually goes in to my dedicated server box, where it is hammered on constantly until it dies. I'm not sure why so many people are jumping at me as if to say I'm not taking care of my disks, since the shortest period of time I've killed a drive in was over 6 years, which is more than 3 times longer than the warranty lasted.
