Wow, not what i expected....

Answer - (i stole this but i dont care)

Lots. Lets start with the fact that hard drives store their information, magnetically on platters inside them. Change the orientation of the magnetized particles on the platters with a magnet and you change the data stored on the drive. Read write heads inside the drive do this for you in an organized fashion, something your PC understands. An external magnet applied to a drive scrambles that into information the PC can't understand. Basically, wiping out anything stored on the drive.