HDs can fail as a PC is turned off, can fail due to ware, can fail if a force of more than 5gs is applied, can fail if you look at them the wrong way, can fail if you give them the wrong beer, can fail just for the shear hell of ****ing you off ... they fail when you are just about to do a back-up that you have put off for the past month and eat all your data just before dieing.
Beware the "click of Death" ...
If you have it, enable SMART in the BIOS ... it is very good at detecting some failures and give you an edge.