Android File Transfer cannot access device storage

I am using a Moto X (2nd Generation). I have a recurring problem that occurs when I plug my phone into my Mac to transfer files (music, etc). When I plug it in, it is unlocked and on. However, Android File Transfer comes up with a message that says: Cannot access device storage And I am not able to transfer any files. I tried plugging it in again, locking and unlocking my phone, no luck. Any solutions?