In researching this week's Fun Friday topics,
I came across this article on Lifehacker
that talks about how to safely delete data from a hard drive. I agree with what it suggests, but when it comes to my personal hard drives, I use a much more direct solution. The tools in that article are good, but nothing beats real, physical damage.
When I'm ready to retire a computer, I strip the hard drive out of it if possible. In newer computers this isn't always so easy, but keep in mind you're probably retiring the computer so have at it. Use a crowbar if you have to. Then I put the drive in a paper bag, to limit the parts from spilling out, and I go to town on it with a 12-pound sledgehammer. I don't stop until the drive sounds like a salt shaker when I shake it. If there's nothing but dust inside the case, no one is going to be able to retrieve anything, period.
This can also be great fun if your computer has ever caused you pain and anguish. I highly recommend it.