The first thing when you record images on a flash drive in a FANUC robot, please test the usb manually by doing an AoA on it, copy files manually etc.
After that if it's not working, then try to format it in your PC and again in the robot, then try it again. In my 8 years experience working with FANUC robots, I found that kingston dtse9 8 or 16 gb works the best with all the controllers that have usb port.
I have stocked around 50 kingston usb drives just for using with FANUC robots