Hi bugger1,
try this workflow:
1. create folder 'map' on your sd card.
2. save your maps in this folder (names can be: 'g1.img, g2.img, etc.etc). if your maps are locked save unlock files in this folder as well (g1.unl, g2.unl etc. etc.)

This works on my nuvi 3790T, the only diffrerence is i'm using 4gb sd card
Regards