GJA utilities allow to write on external card ( even if there is no right to write on external card ) into folder :
Android/data/com.gja.nav.utility/files/...
So, I have iGO folder on internal and .fbl into
Android/data/com.gja.nav.utility/files/iGO_content/content/map ( + 3 x map ).
Added in sys.txt :
[folders]
has_secondary_root=1
android_secondary_root_path="/storage/9C33-6BBD/Android/data/com.gja.nav.utility/files/iGO_content"
In MapChanger the path is : /storage/9C33-6BBD/Android/data/com.gja.nav.utility/files/iGO_content