I was trying to load an OpenStreetMap based map into a Garmin Zumo 220 (motorcycle optimized GPS), and I was running into problems. I could generate the map for the appropriate region from http://garmin.openstreetmap.nl/ but when placing the final gmapsupp.img file into the "Garmin/" directory on the device, the system software would never find it. It was not visible under Tools -> Settings -> Map -> Map Info. Only the original base map showed up.

I tried replacing the ".System/gmapprom.img" with the gmapsupp.img and found that the map was then picked up (but incomplete since I hadn't selected the entire continent due to space constraints on the GPS). So, the file seemed good, but why could I not augment the existing map?

I finally found the answer in:

https://forums.garmin.com/archive/index.php/t-73886.html

There it was described about the "Garmin/GarminDevice.xml" file that defines where it expects to find files. Reading that file revealed that "gmapsupp.img" must be placed in the "Map/" directory, not in the "Garmin/" or ".System/" directories.

Sometimes even simple things can take a while.