Wiping with a magnet does work, though I don't know how effective a permanent magnet would be - very powerful electromagnets are usually used. Even then, the recovery techniques can be very sophisticated if someone is determined enough, and the original data can be recovered even after overwriting with other data. I suspect that magnetic wiping would still leave a bit of a trace that is recoverable. If you want to be certain, then physical destruction by sanding the platters or breaking the platters is still the most certain way IMO.