I’d suggest calling 1-866-SANDISK and asking if Slotradio cards can be resold.This is just a user forum.
I doubt it’s a PC/Mac thing. USB Mode is how music goes on and off the Sansa–has nothing to do with the slot.
But it may well be that the Slotradio card is somehow coded to work only with its first unit, or a limited number of units.
