Disk drives are extraordinarily resilient. I've seen one that had been run over, dropped in a toilet and then thrown out of the window of a seven-storey office block. It looked unusable. Kroll Ontrack managed to recover virtually all the data on it by drying it and taking it apart.
Clam shell casings are very strong and deleting data doesn't work effectively, and passing magnets over the disk isn't sure enough. Even repeated writing of zeroes isn't necessarily effective and certainly takes a long time.
So, if you really, seriously, want to render your hard drive unreadable and haven't used major league encryption of all its contents then what can you do?
Drill the little sucker right through its spindle heart is one answer.