Lately I have replaced a few computers with Windows XP and computers where the motherboard have died. My customers often asks what they can do with their old hard drives.
My main suggestion is that you keep the old hard drives as a backup of your old data in case something was not copied over. If you don’t want to keep the hard drive, I can wipe them completely with a program. This is the most secure way of doing it unless you have a hard drive shredder.
Some of the questions/suggestions that I have heard:
It is enough to use a hammer on the outside of the hard drive? My answer is that it is not enough. The top will not bend enough to destroy the plates. In order for a physical destruction to be somewhat effective you need to open up the hard drive and crush the plates and read-write heads themselves.
What about a sledgehammer? That can work if you have an edge that can hammer through and in to the plates.
Can I shoot it? Yes, you can, but be aware of bullets that might be coming around. You are shooting at a thick metal plate and the bullets could fly in all directions.
What about a strong magnet? An industrial hard drive shredder/magnet will do the work, but there aren’t many of those around. A home magnet will not work.