I met an attorney today who is the president of a digital forensics firm, about what he does and the best way to destroy data. He finds data for people to use in trials. The Dept of Justice brought him a computer that had belonged to a senator from Texas, and it was melted. The FBI came and the senator tossed it right in the fire. It was fused and he got clearance to open it with a hacksaw. He has a dust free room with all sorts of gadgets right in his office, and he salvaged the disk from the hard drive and made an exact copy. . . got everything 100% data recovery. He can copy your whole drive and the drive will never know, there will be absolutely no trace.
I was asking him the best way to destroy hypothetical incriminating data on my computer and he said I really can't. The Department of Defense standard for destroying data is to rewrite the entire drive with 1's & 0's, reformat it and do it again 6-8 more times, or to burn the actual disc (platter) inside the hard drive. Turns out that when you delete something it doesnt get deleted. It just tells your CPU that that file name is available again and the space where it was is now available to store new data. Which data stays and which data get covered over is anyone's guess; and even when you defragment your drive, it covers some data and leaves others. Plus even if its been written over once he can still find it most of the time. And multiple versions of files remain on the drive, so when you save a word doc the old one stays, the new one doesnt over write it.
He said I could buy 2 brand new top of the line laptops for what it would take to custom rewrite mine and that if I really wanted to get rid of the data I should take the drive out and hit it with a hammer until the platter is cracked.
Now ya' know.