I have several SD cards and USB flash drives that are often laying around my work space. Do I have to be careful to keep magnets away from them? (This paranoia is showing my age - I remember needing to make sure magnets never got near my precious 3.5 inch floppies...)
No, unless the magnet is REALLY strong (see below quote). There is not enough magnetic material in them.
"A magnet powerful enough to disturb the electrons in flash would be powerful enough to suck the iron out of your blood cells"
If the above quote is the case you should not be working where you do.
It is also a big myth that normal magnets can ruin harddrives.
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.
But those floppies, don't let them near your magnets.