On the eTrex Vista HCx; Waypoints, Tracks and Routes get loaded to the GPS it's self. Maps get loaded to the microSD datacard, as well as POI files and the card is used for track logging. If the GPS software is current, it will hold up to a 32GB card, but anything over a 8 is overkill. A 4GB card seems like the max needed, but once formatted, it will hold less than 4GB. The unit will format a card without a computer, you might want to try that. Also, you might want to make sure the card is seated correctly. You can put the GPS into USB transfer mode to mount the datacard to the desktop.