Moto G (3rd Gen) on Marshmallow (Android 6.0)
My wife has a Moto G and really likes the phone. She was needing to move and backup some pictures and ran into an issue where it would not work.
On this post “acjaveline” gave the solution that worked for us. It was:
This is a known issue with the Marshmallow update on G3 with SD card set as portable storage (users who switched to internal storage seem to have no issues)… Disconnect USB, then Go to Settings, Apps, and Show System Apps, then find External Storage and Media Storage, and clear data and cache on each one and reboot. Give it 5 min after full boot up to rebuild media databases and connect to USB and select MTP, should be good to go.