When I used a USB cable to the phones, an S5 and now my Note 4;
the Windows File manager can only see certain directories and files on the phones.
it will show all of the main directory folders, but it says "empty" when you try to view the folder contents.
It has always been that way, IIRC.
Android does not give Windows permission to see very much.
Downloads, yes
ringtones, yes
the extSDcard yes, mostly. Not always, some folders it is picky about... with 4.4.4 it was not so bad,
now with 5.1.1 nearly impossible....
so, I quit using the USB cable, and only use the ASUS File Manager.
It copies to/from the PC via WiFi just fine.
Obviously on the PC, the folders you want it to see, must be "Shared"
I cheated with that. I just made the entire 'C:' drive shared....
as I am the only one who ever used this PC, it is not a problem.