If your Behringer UFX mixer shows “nac” when inserting a usb you need to move the switch in the back to the middle position “Stand alone” mode. If the mixer is in usb or firewire mode the standalone mixer will not function.
Once in stand alone the message “FSE” (or maybe “FOR”) means the drive needs formatted. Press the flashing green “Play” button to format or the Stop button to cancel. I’ve not had much luck with this though. I remember it worked in the past so I am curious if newer, larger usb drives are the problem. Update: large drives are no a problem. I was able to record to a 128GB usb thumb drive. I had many drives that would not work after formatting different ways on both a mac and windows computer. I think the issue is some thumb drives are of very low quality. If you are using randing old drives you had laying around they may be on the verge of failure. Just a guess…