My experience quoted was with the EU DEM in Map folder on microSD named D2249750A.IMG and of course it works [and will work even if named Mickey_Mouse.img for that matter] being as it's in the Map folder. As I said, it was selectable/de-selectable individually in the MyMaps menu and the overriding DEM disabling in Map Layers. 'Map Layers' is the same whether reached via Settings>Map & Vehicle>Map Layers or by pressing the 'layers' symbol on the secondary map page.