[ars]

I agree that it makes no sense. A magnet outside the case is not going to do anything to the platters.

kees99 posted a possible explanation, where the magnet moves the voice coil, which ends up destroying the drive by writing data in the wrong places.

[dpedu]

This can't happen either. Early hard drives (and I mean early: the 80s) needed precise head alignment (in respect to the how far the head is from the center of the platter) in order to read or write data in the correct location. If you've heard of "low-level" formatting, that's when a on-platter layout unique to the head and head movement mechanisms of the host drive is created.

Modern drives, however, are designed to "follow" the tracks that are cookie-cutter imprinted onto the platters. We use this design now as it's far easier to design a head that reads tracks and makes slight adjustments to stay on track, vs a head that needs to move to an absolute exact position.

In other words, modern drives are designed to combat this exact problem.