A little for your information everyone
I installed the cyanogen mod 12 which claimed to have usb mass storage enabled on it.
It does and this works perfectly and I am very pleased with the performance. Just a simple option when you connect the device to change to usb storage.
I'm running CM12.1, and have been trying for weeks to get true UMS (usb mass storage) to work, like it did back on my blade & Galaxy S2....
I've searched out apps and mods to enable it, and so far, no joy.
For me, CM12.1 "seems" to offer UMS - When I plug the phone in to my PC via usb cable and it mounts via MTP, I can then navigate to Storage Settings, and from there access a menu that gives the options for MPT, Camera, and UMS.
However, when I enable the UMS option, I can't actually read or write to the SD card in windows (using Win-7), so it's like the option is there, but disabled...
I see what I would expect (namely, the phone storage disappears from Windows Explorer, and the SD card appears as a flash drive), but when you navigate to the "flash drive", it appears as an empty drive with no data on it.
If you'd be so kind, would you explain in excruciating detail exactly what you did to enable true UMS on the S4 using CM12/12.1?