nuvi 6x0 can only see one image on SD named x:\Garmin\gmapsupp.img. It can read both FAT16 [>2GB] and FAT32 cards [4GB and over] but the gmapsupp.img file size must be less than 2GiB.

@bizzo
Some additional things to check:
To ensure that the SD is being recognised by the device, on the main screen hold the 'time' in the top RH corner for ~6 seconds to reach a hidden menu. Press 'Next' twice to reach the 'Data Card Test Page', wait for a few seconds and the card's info will show if the card is being seen.

To check that your map is ok, open it in [Only registered and activated users can see links. ], press Info or Details Tab to see if any errors are shown or it's locked. Run [Only registered and activated users can see links. ] on the img to be sure it's not locked.