A possible failure when upgrading Firmware using UCM/USB may be plugging in of 2 or more USB interfaces into the Computer doing the upgrading.
This sometimes causes a loss of communications after some time. It may work well for a long time, but may cause problems at other times.
It is best to remove any other USB devices from the computer before trying the firmware upgrade..
This applies to upgrading by firmware upgrade cable as well as by direct comfigurator upgrade.
We had a situation where firmware upgrade repeatedly failed using both methods. Then we noticed that there was another USB plugged in After removing the other USB, the firmware upgrade was immediately able to work