I have a GPS map 66st, which has over 2000 waypoints and 150 tracks in it. When I connect it to the Explore app, it sits for a long time, and then the next thing I know, almost all of my waypoints and tracks in the GPS are gone. I've tried deleting the tracks, and the waypoints synch just fine. As soon as I put 10 tracks into the GPS, the synching stops. These same track synch just fine from my Fenix 6X pro.