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.
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.
But those floppies, don't let them near your magnets.