I did this aswell with my 40x flashed to 48x i was the crazy tester and flashed my 40x with 52x firemware and i to had a dead drive
speaking of dead drives i have had a lot LOL
but i managed to ressurect them all.
this is no diffrent to a dos flash.
go into your bios and disable your drive put it at disable or none ignore auto.
then go into dos flash it and it should work im 100% shure not 99% / 100%
then go back into dos and enable your drive.