Hard Disk Drives (HDD's) are one of the most impressive and important electromechanical devices ever created. When I think about it, it is really amazing that these things actually work, let alone work so well! These disks must quickly and precisely position heads slightly above very narrow tracks on rapidly spinning platters. If there is an error in the angular position of the head, it will not be positioned above the correct sector and therefore cannot correctly read or write data.
