One random write over the entire hard drive should be enough to make any useful data unrecoverable. It seems to be enough in controlled studies, when the researchers know what they are looking for and where it is on the hard drive: http://dkn255hz262ypmii.onion/index.php?topic=99520.msg699299#msg699299 It would be even harder for someone who doesn't know what is on the drive. DBAN does three writes, two of random data and the last one is a zero write to make it look like the hard drive was not secure erased. That's the recommend tool for this job. http://dban.org