My Nuvi can detect jpg but cannot detect map file (.img) in my sd card.
I tried to put 2-3 map in sd card but no one of them listed in map info.
At first I thought it cannot detect the card, but in picture viewer I can see all jpg loaded in that card.

Update:
Try using different card, regenerate new map, 1 map, 2 maps, also give same result.
