How do I copy files between OS X and Android 6 over USB cable? I want to copy files from OS X (El Capitan) to my Nexus phone (which uses Android 6) and vice versa. And I just mean copying locally, i.e. over USB cable, I want to avoid overcomplicating things with upload and download through intermediary servers.

In the past I used Android File Transfer for this. But that was with a previous OS X version, and my old phone with Android 5. I tried again, but it doesn't work anymore, it keeps saying:


  Can't access device storage.
  
  If your device's screen is locked, disconnect its USB cable, unlock your screen, and then reconnect the USB cable.


The screen is most definitely not locked, and dis+reconnecting the USB cable (either end) doesn't help.
Pull down notification menu.
Tap "USB for charging / Touch for more options"
Tap "Transfer files (MTP)"