Secondly, if the red LED on the back of the board is on, that means the
board is powered. When your computer communicates with the main
board via the USB cable, the yellow LED on it will flashes. For example, it
will flicker when you burn a “hex”file.
Then Micro: bit main board will appear on your computer as a driver
named “MICROBIT(E:)”. Please note that it is not an ordinary USB disk as
shown below.