I have a PA-100 Pro using firmware 3.1.1.18 and there is no menu option on the small screen to put the device in USB drive mode !!
The device has been reset to factory settings, SSD reformatted, SMB and USB drive options turned on in the app. Nothing will make the option appear for USB drive mode in the small screen menu.
A major bug which needs urgent fixing as I am not able to connect the device using USB C