I agee with you, but when I connect my 3760 on my pc I do not see the maps (img) installed on internal memory.
On my microSD card I have garmin/gmapsupp.img , installed by me.
I copy O.F.R.M. Geotrade 5.21 LAT on card and I renamed as gmapprom and gmapprom1 and I can't see the map; also O.F.R.M. Geotrade 5.21 LAT alone ( as gmapprom) on card don't work.