Or use secure empty trash. I tried the reformat but it didn't work for who knows why so I dumped everything in the trash and did it secure. It took almost 12 hours though. Remember to flush all of the cookies and passwords off the web browsers
actually a quick erase and reformat (make 2 partitions, change back to one)will do-take 5 minutes-or just reinstall OS X
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I think the DOD erases once and then cuts the HD into halves, then quarters with a bandsaw then tosses into incinerator-makes it pretty hard to retrieve data when they melt significant parts of the the HD, rather than the myth of them erasing it 100 times of whatever
but dont quote me