Originally Posted by Village Idiot

When you delete things from your harddrive, they're not really deleted, the flags are just removed. The information doesn't get truly deleted until it gets written over.

When the flags are removed, it shows that the disk has erased it and that the drive has that much more space free.

It's how actual hardware works vs. how software shows what's happening. To actually erase a whole drive, you'd have to degauss it or use a formatting utility that writes "nothing" over the entire drive.

Time Machine may be able to bring back files from that information...It's just my speculation.