Apple - How do I copy files between OS X and Android 6 over USB cable?
- Pull down notification menu.
- Tap "USB for charging / Touch for more options"
- 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".