I have had this same 'Cannot mount volume' message when trying to swap files
from my Windows PC to the Ubuntu box with a USB stick..

Turns out the problem occurred when I simply 'unplugged' the USB stick from
the Windows PC..

I doscovered if I first dismounted the USB stick using Windows 'Safely
remove hardware' option t, Ubuntu accepted it without problem.

Maybe if you dismount the USB reader before removing it from your computer,
it will continue operate as expected?