You can exit EmulationStation and then, from the command line, runsudo umount /media/usb0
If your USB drive is mounted on another folder than /media/usb0, substitute /media/usb0 with the proper folder name.
It's apparently the best solution... what i have do:
F4 to exit from ES.
mount (to found corrent usb)
used sudo umount
copyed all my news roms from pc and now also the image snap and some gamelist.xml from my personal system
type exit from console to restart es and....
all now works whitout any issue.
Thanks for all.