I ended up on this thread after my Samsung Galaxy S5 did the same thing. I'm getting rid of the phone soon, but thought I'd provide some insight for anyone else having this problem. On the phone, slide down from the top and there should be a notification that says "File Transfer - Tap here for more options." Tap it and options will come up to change the phone from the default Charging (really?? I've never plugged my phone into the computer to charge, only to move files. So stupid) to File Transfer. This allowed me to move/copy my pictures to my backup folder.