@berrinjames
Safer to put 'unofficial' unlocked maps on an sd/microsd. Telling us your unit model would help, but all nuvi models will read a map image named gmapsupp.img placed in a folder named Garmin in root of card.

Some models (e.g. 2xx/7xx series) will also read a 2nd img named gmapsup1.img in there. Later series (eg 1xxx, 2xxx, 3xxx & 30/40/50) will read multiple maps with anyname.img (e.g. CNNT_SAM_2013.30.img) in a root folder named Map (case sensitive).