What is degaussing?
Degaussing is a sanitization method that exposes magnetic storage media to a powerful magnetic field, scrambling the magnetized regions that hold the data. The name comes from the gauss, a unit of magnetic flux density. A degausser uses either strong permanent magnets or an electromagnetic coil that releases a brief, intense pulse. It works only on media that store data magnetically: hard disk drives, backup tapes such as LTO cartridges, and older floppy disks.
How degaussing works
A hard drive records bits as tiny regions of magnetization on its platters. Each medium has a coercivity, the field strength needed to change that magnetization, measured in oersteds. Dense modern drives use high-coercivity media, so a degausser has to be strong enough for the media being erased; an underpowered unit can leave data readable. U.S. government users select models from the NSA's evaluated products list for degaussers, and other organizations often use that list as a reference.
The same field destroys the factory-written servo information the drive relies on to locate its tracks, so a degaussed modern hard drive cannot be reformatted or put back into service. LTO tapes carry factory servo tracks too, so a degaussed LTO cartridge generally cannot be reused either.
What degaussing cannot erase
Degaussing has no effect on media that do not store data magnetically:
- SSDs, NVMe drives, USB flash drives and memory cards, which store data as electrical charge in flash cells
- Phones and tablets, which use flash storage
- CDs, DVDs and Blu-ray discs, which store data as physical marks read by a laser
Hybrid drives (SSHDs) need extra care: degaussing erases the platters, but the flash cache inside the drive can still hold copies of recently used data.
Degaussing in a disposal program
NIST SP 800-88 treats degaussing as a purge method for magnetic media when the degausser is matched to the media's coercivity. Its 2025 revision narrows that guidance and does not count a degaussed drive as destroyed, even when the drive no longer works. Degaussing fits two situations especially well. A failed hard drive cannot be overwritten because it no longer responds, but it can still be degaussed. Backup tapes past their retention period can be degaussed in bulk far faster than they could be overwritten. Because the drive is unusable afterward anyway, many organizations degauss and then shred, which also gives visible proof of destruction.
Verification works differently from overwriting. You cannot read a degaussed drive to check it, so the program relies on a degausser rated for the media, periodic testing of the unit and a log of every serial number processed, often backed by a certificate of destruction from the disposal vendor.
Degaussing vs. overwriting vs. shredding
| Method | Works on | Media reusable? | NIST level |
|---|---|---|---|
| Overwriting | Working hard drives | Yes | Clear |
| Degaussing | Hard drives, tapes, floppies | No, for modern hard drives | Purge |
| Cryptographic erase | Encrypted drives, including SSDs | Yes | Purge |
| Shredding | Any media, with a particle size suited to the type | No | Destroy |
Degaussing on the CompTIA Security+ exam
Degaussing falls under SY0-801 objective 4.2, in the disposal and decommissioning stage of asset management. Wording such as expired backup cartridges or dead hard drives that still hold patient records often points to degaussing. Any answer that degausses SSDs, flash drives or optical discs is wrong, because those media are not magnetic. Degaussing is also wrong when a hard drive must be returned in working order, such as at the end of a lease. A+ Core 2 (220-1202) objective 2.9 also includes degaussing among its data destruction methods.