Hi ;=)
I rebooted with the usb drive plugged, i did not reboot on purpose on it, but it is possible that it used it as 1st drive being read (i don’t remember the exact setting of the bios).
Del is the key i usually need to enter Bios settings, when i boot and press on Del key i have a black screen and 2 things written on it :
“Enter the menu” on the bottom left side and “A2” on the bottom right side of the screen.
I don’t know if it’s the bios or another menu, i’m not sure so far.
I’m thus not able to enter the bios as usual but there maybe some good tips/tricks that could be useful in this case , i hope so 
btw : i had upgraded another bios before on another MB, the process went smoothly. This time it was similar but i thing i cut out some files too quickly, i was trying to gain some time, but i was wrong.