the government uses electron microscopes to measure the pitting on the hard drive platter to analyze the data that was written there.
it's like pressing down when you write on a pad of paper. after you tear off one sheet, and write on the next, then tear that sheet off, the sheets of paper under that will still bear evidence of what was written on the top "layer"
Kevin Mitnick got rid of his hard drives (according to the book by Dan Verton, "the Hacker Diaries") by throwing them into a swamp. they were never recovered.
~Branson