Contrary to popular belief, magnets, degaussers, and other x-ray or magnetic techniques are not the answer to wiping your computer's hard drive clean. In fact, drilling, smashing, torching and other external damage doesn't do the trick either, as those aggression releasing techniques can still leave parts of the platters in decent shape to retrieve data through forensics. Of course, chances that someone will take to using forensics to get a hold of your hard drive are minimal (unless you're a criminal), but you can never be too safe.
German encryption technology specialist Pointsec recently tested hard drives bought on eBay to see if they still carried data and discovered that seven out of every 10 devices it tested still bore readable information. That study followed similar research published in 2003 that found only 12 of the 129 working computer hard drives they bought in secondhand stores and on the auction site eBay had been adequately cleansed of sensitive data from their previous owners.