Moto G external storage failing to show on Mac and other USB issues
I showed this to a pal of mine who uses the same setup and he had the same issue. He discovered the symbolic link to the SD card was incorrect and of course not editable for non-rooted devices. He found a fix:-


Eject the SD card from Setup>Storage
Shut the phone down
Remove the SD card
Restart the phone
When running, put the SD card back in.


This seems to rebuild the symbolic link and now I can see its contents again via Android file transfer - hope this might help others.