Actually, I've tried an AC degausser/demagnetizer on floppy disks: it's just a big coil. Surprisingly, some floppies actually came out intact! (Thanks Radio-Scrap!)
Again though, because the voltage changes (AC), it creates a varying magnetic field. The variation in magnetic field can be translated in some unit of measure (I don't remember which, that part of physics was a long time ago!). A fixed magnet's field doesn't move, unless you move the magnet, or move the object near/around the magnet...
c'mon... won't you try to trash that HD for us? In the interest of science...
