Apple - How do I copy files between OS X and Android 6 over USB cable?

  1. Pull down notification menu.
  2. Tap "USB for charging / Touch for more options"
  3. Tap "Transfer files (MTP)"

In Android 6 there is new connection mode - charging from usb. In this mode you can't transfer any data. I faced it on my Nexus 5 with El Capitan.

Solution was easy - change connection mode to data transfer. I used this mode before, but with Android 6 phone set default connection mode to "charging".