Contrary to popular belief, magnets are very unlikely to cause hard drives (or even floppies) to lose data irretrievably.

Whilst a bulk eraser can be used to 'wipe' a floppy so that it can be re-used as a 'new' disk, the data could be retrieved - and often has been for forensic purposes.

The only sure way to destroy a hard drive is to take the lid off and pour acid on the platters...! A hard disk platter is usually made from aluminium or glass with a magnetic coating, and destroying that coating is guaranteed to do the job. Once upon a time they used to be steel discs, but that is rare these days.

If the platters are glass, of course, you can just shatter them - but Murphy's Law says that the one you want to destroy won't have glass platters!!

It isn't enough to damage the heads or electronics, as data retrieval specialists take the platters out and fit them into another identical drive. Okay, there are usually other issues which have to be addressed, but that's it in principle.