you must create a Map folder in the internal memory and rename the map as anyname.img but not gmapsupp.img. It wont see it in .System .
Map folder in sd mustnt be in Garmin folder but in root folder, if 3790 doesnt see it it's locked.

Right paths and naming in 37xx and other new Nuvi devices example:
Nuvi root/Map/CNE.img
SD/Map/CNE.img
SD/Garmin/gmapsupp.img (just one map in Garmin folder can be seen by nuvi in 3790)
If any of these work ,the map is locked, check your .img with GmapTool, drag it in GmapTool and check the log-