Hard drive shredder:
A device that physically demolishes the platters of a hard disk to ensure that their contents can never be recovered. Hard drive shredding services may be offered by service companies that shred paper and microfilm.
Software that permanently erases data on a hard disk by overwriting all the sectors with random characters. The normal deletion of a file only frees up the disk sectors involved, but the original data are still there. In fact, there may be magnetic residue from bits that were written several cycles in the past and that have been written over numerous times with new data.