I don't know all the ins and outs of this, however it has happened on both the sites I look after from time to time. To troubleshoot, I have tried reconfiguring Workstation backup, and found the system could not find the USB drive. Most of the time, I fixed the problem by removing and re-inserting the USB drive, and testing by configuring Workstation backup. If all else fails, reboot (but you should not have to).