The best way to wipe hard drives clean is still to use a big ‘ol lunkin piece of magnet, so researchers say:
Erasing a hard drive usually takes hours using special procedures that repeatedly scramble information on a disk drive. Still, given unlimited resources and time, special magnetic snooping techniques can often recover at least some of the original information.
The researchers concluded that permanent magnets are the best solution. Other methods, including burning disks with heat-generating thermite, crushing drives in presses, chemically destroying the media or frying them with microwaves all proved susceptible to sensitive, patient, recovery efforts.
Thinking of selling the old computer on eBay? Scrub your drive clean first — lest there be known, what you have seen on the Internet, what you have bought, and the credit card number used.
Full article here: http://www.darkreading.com/document.asp?doc_id=97378