This short explanation will tell you the best way, and usually only way, that people permanently destroy the data that is on a hard drive.
A lot of idiots will tell you that you can destroy data that's on your hard drive by using certain programs. I wouldn't bother with the hassle when hard drives are so inexpensive these days. With this method, a person would need to buy a new hard drive.
First they need to take their hard drive out of their PC or laptop. I'll be using the hard drive that was in my laptop for this explanation.
When they pull it out, it has some mounts on it, like the ones in this picture. They are those metal things that run down the side of the hard drive and have little blue plastic ends.
After taking those off, they need a star key to remove the outer-back case. After taking off all of the screws, they simply pry up on the case. I used a flat head screw driver for this one.
At this point, the disks can be seen. They are very reflective and hold the evidence that so many detectives and federal agents just love to seize during a search warrant. They look like this...
They simply place the whole hard drive into their sink, and place a paper towel over the disks(these will shatter). Then they use the flat head screw driver to apply pressure to the disks until they shatter. They would keep moving the screw driver around until it's just a nice glass mess. Like this....
Then they would pour these glass remains into a paper bag and toss it discretely into a trash somewhere.