I recall a post a while back that said Navigator wasn't reading maps on the SDcard. I have a Samsung J5 on android V6.0.1 and Samsung tablet SMT-116 android V4.4.4.
Early this week due to space shortage I thought I'd try both on the SDcard. All I can say is that it seems to be running FINE on both. Due to that post I hadn't dared risk it. Maybe the problem was android V5? (V6 upgrade was last week).