Would a strong magnet have any effect whatsoever on a thumb drive (I'm assuming not) or on an SD card? It seems unlikely, but I'm hoping someone can give me a definitive answer, since I'd rather not find out the hard way that it actually can. Assume the magnets are powerful industrial magnets, if that makes a significant difference to the answer.
I've tested many card with my 1.5Kg rare earth magnet, so I can bet that magnets have no effects on flash cards or USB pen drives :-)
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.
Assuming you are talking about ordinary magnets, no.
If you are talking about the field strengths found in an MRI machine or a fusion research device, things start to get weird - there's the hall effect, potential for induced current due to movement or field changes, even potential for mechanical distortion and having parts ripped out of the assembly.
Magnets will not, according to the gurus at PCWorld, affect your SD cards, since they are just flash media (like thumb drives).