Speculating doesnt help TS much though.
He should just try swapping drives, if it boots he's going to install updates anyway and if they behave as you describe he will not have problems.
Well, I know for sure that I have had instances where it has not worked, which certainly means I can't claim "it will work." I can say "It will work under these conditions" which is exactly what I said.
You can pretty much always boot them, but things can go downhill from there.
Maybe i can restore the dmg file to a usb flash drive?
That should work, I've done that before.