Physical destruction is best
This will put the data beyond reach of anyone except a three-letter agency (and probably also the agencies).
0. Make sure it's a magnetic disk not an SSD or hybrid
1. Smash the electronics board with a hammer (probably optional, but satisfying).
2. Drill several holes in the top of the HDA
3. Put the disk in a tray and pour xxx-cola into one of the holes until the HDA is full, then more to cover it. (The multiple holes are to let the air / gas vent).
4. Leave overnight to dissolve the magnetic domains off the platters. (You know what it does to a tooth, right? )
5. Throw it away.
For an SSD you need an incinerator. (Or a decent bonfire, and utter disregard for the anti-pollution regulations).
Biting the hand that feeds IT
