Map visible in BaseCamp

At "Files" tab add a map.

At "Write" tab set Mapsource flag to 0.

Click at "Write all" button"

Value 0 for Ms flag makes BaseCamp or HomePort read map from device memory. Value 1 is set by Mapsource and Mapinstall. BaseCamp and HomePort don't read maps with this value, they assume map is installed on PC.