i found the 50 min wait with engine on for the USB u/g process to complete the worst part, hangs on the map/nav update forever, and needed 32GB USB to fit it all on. so when i updated in June (Rel_UF.20.14.70 EU), i did it when i had a long drive planned, and then when unit rebooted, i managed put my MP3 music USB back in. All good.
Just finished the up load into the car. I didn't start the car, pushed the on button twice, turned off the HVAC system put the thumb drive in and followed the prompts. started at 1:55PM, finished at 2:35PM.
Started the car afterwards and let it run for a few minutes.
I sat there and looked through the owners manuals, good thing to do.
I just tried the update. It failed. I tried again, reinstating the USB drive, but still nothing (no percentage countdown, no messages, just a blank screen). Eventually, the software failed box came up.
I installed the update yesterday. Much to my annoyance I had the same "USB was unplugged..." error message to start and every time I pressed the "Retry" button. After getting a bit frustrated by the problem, I turned the car off and on again. This time the head unit was able to install the update without any problems. Took about 25 minutes to update. Give that a try if you run into the "Unplugged" problem.