I have a Nuvi 1470T (Taiwanese version). With the above method I've created a folder "map" on the device AND on the SD card. Then I put a couple of unlocked maps (gmapsupp.img) in those folders. They were both recognised and worked perfectly.
However when I tried replacing those maps with another (gmapprom.img) it didn't work. Do I need to unlock those gmapprom.img files? Or is there another method to install those maps?