"An unexpected error has occurred"

Did it have a name or number for the error? If it does, try plugging it straight into google to see if anyone else has had it before. I'm not certain, but my best guess would be that it might be trying to read something other than the USB with the bootloader on it first and it doesn't like that. If nothing else is plugged in, it could be that there is something wrong with the USB port or with the flashdrive not communicating with the motherboard.