i am using edge 800
I name the maps instead of gmapsupp.img
eg:
gmapsuppThailand.img
gmappsuppIndonesia.img
gmapp suppMalSing.img

All the files placed in the garmin folder of micro sd card.
The device can read all these maps without having to join the maps using gmap tool.
Wonder if it's a new feature in newer garmin devices.
IOh, btw, it has a basemap already preinstalled.
Perhaps this helps?