What is Degaussing?
Magnetic storage media have a surface applied to the device that can accept a magnetic field or domain. In the case of tape this is applied to the underlying plastic tape, in an HDD it is applied to the surface of the metal or glass disk. The read/write mechanism then creates a specific pattern or domain in a specific place, that is the data being stored. The place (on the device) is remembered so the read/write head can be directed to go back and find the specific data when needed.
The magnetic field created, both the ‘remember where the data is’ and the actual data itself, has a strength. That strength, or technically, the field’s resistance to being erased, is measured in units of Oersted and is called the coercivity of the field. Modern storage media have coercivity of about 5,000 oe.
To eradicate that magnetic field, that is the data, you apply a stronger magnetic force. One gauss effectively kills one Oersted. To eliminate the data you apply a force measured in gauss, that eliminates the data/field. Modern degaussers apply a minimum of 10,000 gauss in either a magnetic pulse, or through a permanent rare earth magnet.
This degaussing process eliminates the original magnetic field that was the data, rendering the information unrecoverable through even the most advanced forensic means.