Short of completely melting or disintegrating the disk down, there's almost no way that you're going to be able to "destroy" a disk badly enough so that high-level government is not able to somehow ressurect the data that was on the drive. Even the "commercial" degaussers (often used to "erase" tapes and the like) aren't strong enough to destroy everything on a disk so that they can't ressurect it (but are good enough so that "common people" and public agencies have close to a snowball's chance in hell of getting it back) - these devices, from what I understand, are actually limited in their capabilities so-as to make it possible/easier for the government to still be able to read what was once on a tape or a drive.

The way I've had it explained to me in the past (or at least what I remember of it) is that you can think of your harddrive as being composed of tracks as wide as a 20 lane freeway... when the device is written to, a big-huge truck comes flying through there and writes the data along whatever path it takes along this microscopic path (ie. over many lanes of the "freeway," including and that "splashes out and hits the gutters" and similiar). As strange as it sounds, the heads of a drive don't actually ever really traverse the same path along the same track... and even formatting or "demagnetizing" a drive (or writing over it, for that mater) still leaves traces of what was once there. Certain agencies are capable of pulling apart the drive, piece-by-piece, and reassembling it to what it once was... and getting data off of that...

It's really pretty frightening when you think about it.