U.S. Department of Health and Human Services
Before a system component is disposed of, it must first be sanitized which destroys all data by either clearing or purging it. Cleared data cannot be reconstructed using normal system capabilities but can be reconstructed with commercial software. Purged data can also be reconstructed, but only by using expensive techniques.
Purging techniques include overwriting, degaussing (demagnetizing), and destruction. The method you choose depends on the sensitivity level of the data:
Overwriting is accomplished with special software used to overwrite every bit in every sector of memory.
Degaussing is more destructive and involves physically destroying the magnetic image
Destruction is the most reliable technique since the media is taken to an approved facility for incineration or application of an abrasive substance.
For additional information, see NIST SP 800-88, Guidelines for Media Sanitization
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Disposition Phase Security
Media Sanitation Options