Since upgrading my Moto G 2nd gen to OS 6.0 I am having issues with USB and file transfers when connected to 3 different Macs with different OSes which all worked OK before. Sorry this is a bit long...
When I plug it in, Android File Transfer (AFT) starts up automatically but I get an error window saying 'Can't access device storage'... I find the phone is in USB charging mode only. If I change it to MTP, AFT does then show the internal storage files, but will not show the contents of the SD card. I have also tried PTP mode (which would be more useful) but iPhoto does not show any of the (new) photos, which it used to before the upgrade.
Additionally sometimes the Mac appears to lock up - no keyboard or mouse control - I suspect the USB circuitry is getting locked up - even unplugging USB does not clear this. The only 'fix' is to hold down the power button to reboot.
I can see the SD card contents on the phone and can copy files on/off with OTG but this is a bit of a pain.
I've not read of anyone reporting exactly these issues.