Can a magnet erase a hard drive?

Almost certainly not in the way the story goes. In one published test, a magnet retailer held large neodymium magnets against a running hard drive and could not change a single file. SSDs, SD cards and USB sticks are flash storage, which is not magnetic at all, so a magnet is the wrong tool. The one real hazard: a powerful magnet held against a drive can damage it, and damage is not erasure.^[1][2]

A magnet shop tried it, and the drive shrugged

K&J Magnetics sells magnets, which makes its own failed attempt good evidence against the trick. It filled a 30 GB drive with files, ran it from a Linux CD, held neodymium magnets of many sizes against the spinning drive, up to a three inch disc, then re-checked every file for errors. Nothing had changed. The company's conclusion was doubt rather than confidence: its results, it wrote, make it doubt that neodymium magnets are sure to erase all the data.^[1]

Why drives resist casual magnets

The property that matters is coercivity, which NIST's glossary describes as a material's ability to withstand an external magnetic field without becoming demagnetised. Media are engineered to hold their patterns through stray fields: NIST notes recording technologies now use higher coercivity, so much that many existing degaussers do not have sufficient force to effectively degauss such media. Real degaussing is an engineered operation, purging magnetic media only when the degausser's strength is carefully matched to the media's coercivity. Against that, a magnet from a speaker is an unreliable erasing tool: the likelier outcome is damage, not unreadable data.^[2][1]

SSDs, SD cards and USB sticks: not magnetic

Flash memory is not magnetic storage. NIST instructs that degaussing should not be used for non-magnetic ISM, its abbreviation for the storage hardware itself, and gives flash storage such as SSDs as the example. So a magnet passed over an SSD, an SD card or a USB stick neither erases nor scrambles it. NIST observes that a degaussing run on an SSD 'can complete successfully, but no sensitive data is sanitized'. The folklore does not apply in either direction: nothing magnetic wipes flash.^[2]

If your goal is making data unreadable

If you want data gone, documented routes work where magnets do not. NIST's glossary defines sanitising media as rendering access to target data infeasible for a given level of effort, and describes overwriting as typically hindering recovery even against laboratory techniques. On flash, though, overwriting has a limit NIST spells out: spare cells and wear levelling make it infeasible to reach every area where data was stored, which is where cryptographic erase, which NIST calls noteworthy for rapid, high assurance sanitising, comes in. Seagate's article on Diskpart's clean command says it will permanently erase/destroy all data on the selected drive in minutes. And half-measures fail: NIST cautions that bending, cutting or drilling a hole may only partly damage the media, leaving portions of the data reachable in a laboratory.^[2][3]

What happens when you delete a file

How long hard drives last