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

Solution 1:

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

Solution 2:

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".

Solution 3:

As far as I know, you need drivers in order to do that. You can download android drivers for instance here. Linux Ubuntu seemingly allows this already from my experience but it didn't work all the time and threw cannot mount errors. Rooted phones also make it easier to transfer files.

People could have problems with the update to Android 6 or El Capitan. You can in that case try re-installing, or enabling developer options (Tap build number in about this device in settings 7 times to unlock developer options, then enable USB Debugging).