if you are booting into the exam environment, it should never prompt you to remove the USB drive. It looks that it might be an artifact on the screen. What happens if you ignore the message and leave the drive plugged for a few more minutes? (eg 5 o 10, to fully give the laptop time to boot).
Ok I tried again and ran into the same issue...screen frozen on the 'Remove USB' screen...I waited for almost 30 mins, but the screen remained unresponsive...
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