[jcrawfordor]

1) Modern research suggests that recovering data which has been overwritten once on magnetic media is probably not feasible due to the high density of modern magnetic storage devices and the use of dynamically aligning heads (e.g. that follow track geometry as they read) instead of absolutely positioned heads (stepper motors) that allow for more error in alignment.

2) Nonetheless, there is an appreciable risk of data remaining on the device due to non-volatile caches, remapped disk areas, and other factors that are not always well understood or disclosed by the manufacturer. Manufacturers have also been found to be unreliable in their implementation of ATA security features (e.g. embedded secure erase). As a result, with few exceptions it is U. S. government policy to permanently destroy all storage devices rather than trusting any kind of secure erasure. The typical NSA-approved method is either to degauss (surprisingly tricky to do right) and then crush, or reduce to 2mm particles with a device resembling a large, terrifying blender.

[jfrunyon]

> As a result, with few exceptions it is U. S. government policy to permanently destroy

And as a result, (probably) Iron Mountain makes bucketloads because we don't just encrypt the drives (let's face it, that would be perfectly acceptable for 99% of US government computers - maybe Top Secret stuff deserves proper destruction).