The USB port of the NI roboRIO uses a standard USB type B connector, which has metal folded to the inside of the port. It is possible for a USB cable to attach itself to these metal pieces and not allow for release.
- Power off your device and disconnect all other I/O (to avoid electrically damaging the device)
- Find a small piece of plastic or metal and use it to clear the small space between the port's inner walls and the USB cable
- Disconnect normally