Editing Degaussing (section)

=== Irreversible damage to some media types ===
Many forms of generic magnetic storage media can be reused after degaussing, including [[reel-to-reel audio tape recording|reel-to-reel audio tape]], [[VHS]] videocassettes, and [[floppy disk]]s. These older media types are simply a raw medium which are overwritten with fresh new patterns, created by fixed-alignment read/write heads.

For certain forms of computer data storage, however, such as modern [[hard disk drive]]s and some [[tape drive]]s, degaussing renders the magnetic media completely unusable and damages the storage system. This is due to the devices having an infinitely variable read/write head positioning mechanism which relies on special servo control data (e.g. [[Gray code#Position encoders|Gray Code]]<ref>{{cite book |title=Official Gazette of the United States Patent and Trademark Office: Patents |date=2001 |publisher=U.S. Department of Commerce, Patent and Trademark Office |page=1977 |url=https://www.google.co.uk/books/edition/Official_Gazette_of_the_United_States_Pa/9Qzw-6c9OeUC?hl=en&gbpv=1&dq=gray+code+magnetic&pg=PA1977&printsec=frontcover |language=en |chapter=US patent 6,201,652 B1}}</ref>) that is meant to be permanently recorded onto the magnetic media. This servo data is written onto the media a single time at the factory using special-purpose servo writing hardware.

The servo patterns are normally never overwritten by the device for any reason and are used to precisely position the read/write heads over data tracks on the media, to compensate for sudden jarring device movements, thermal expansion, or changes in orientation. Degaussing indiscriminately removes not only the stored data but also the servo control data, and without the servo data the device is no longer able to determine where data is to be read or written on the magnetic medium. The servo data must be rewritten to become usable again; with modern hard drives, this is generally not possible without manufacturer-specific and often model-specific service equipment.