For venerable floppies, this statement holds true. We placed a 99-cent magnet on a 3.5-inch floppy for a few seconds. The magnet stuck to the disk and ruined its data. Fortunately, most modern storage devices, such as SD and CompactFlash memory cards, are immune to magnetic fields. "There's nothing magnetic in flash memory, so [a magnet] won't do anything," says Bill Frank, executive director of the CompactFlash Association. "A magnet powerful enough to disturb the electrons in flash would be powerful enough to suck the iron out of your blood cells," says Frank.
The same goes for hard drives. The only magnets powerful enough to scrub data from a drive platter are laboratory degaussers or those used by government agencies to wipe bits off media. "In the real world, people are not losing data from magnets," says Bill Rudock, a tech-support engineer with hard-drive maker Seagate. "In every disk," notes Rudock, "there's one heck of a magnet that swings the head."
Want to erase data from a hard drive you plan to toss? Don't bother with a magnet. Overwrite the data that is stored on the media instead. For flash, fill up the drive with anything, like pictures of your beloved dachshund. Unlike with magnetic media, from which experts can usually recover at least some overwritten data, once new data is written to flash media, the old data is gone forever. To overwrite the contents of a hard drive, try
Eraser from Heidi Computers.