For years, there has been a myth that magnets will damage computer hardware. Users have been instructed to keep magnets of any kind away from any computing products. In the nascent years of computing, magnets could have posed a threat. But modern-day computing has nothing to fear from household magnets.
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Floppy disks, which really aren't used anymore, fell into a category of memory called magnetic media. This was because there was a black film inside the disk that was magnetic, and it was read and written by different kinds of magnets. If this magnetic media was exposed to an external magnet, the data could be lost or corrupted.
More modern forms of portable memory, such as SD memory cards and USB flash drives, do not have magnetic films to record data like their floppy-disk forefathers did. As a result, they are immune to interference from external household magnets. "A magnet powerful enough to disturb the electrons in flash would be powerful enough to suck the iron out of your blood cells," said Bill Frank, executive director of the CompactFlash Association.
Hard Disk Drives
Slightly more susceptible to problems with magnets, hard disk drives still won't break a sweat around a household magnet. Lab- or government-grade magnets can wipe hard drives, but nothing you would be able to purchase at a hardware store is anywhere near strong enough.
The most obvious way to prevent any damage to a computer from a magnet is simply to keep magnets away from your computer, especially when the computer is on and when you are using a strong magnet. Still, you don't really have very much to worry about.
Wiping Memory Devices
If you want to clean out memory that you are throwing or giving away, using a magnet is not your best choice (unless you're still using floppy disks). Flash memory can be completely obliterated by overwriting the data. You can copy pictures, music or any other files onto your flash drive or memory card, and anything that was previously stored will be forever gone. Hard drives are more complicated; data can sometimes be professionally recovered even after an overwrite. Use a hard drive wiping program to clean any hard drives you will be discarding.