Guys, he has a nuvi 50. It isn't an MTP device.

@djdyna
Try this: Make a copy of your GarminDevice.xml. Get a clean microSD/SD or USB stick at least 4GB, create a Garmin root folder and in there paste the xlm file and nothing else. Run MapUpdater and install your map to the 'fake nuvi'. If it installs correctly, copy the map and associated unl/gma files over to your nuvi and boot it up.