Depends on how bad your v78 drive is messed up. The jist of it goes like this, you get the key from you original drive, inject that key into the new drive and spoof the new drive so it reports it's really the original drive. Now all that sounds kinda easy, but since you have a v78 and with it being messed up that makes it a bit harder to do. The FW (FirmWare) chip may need to be removed and dumped with an external programmer if that thing doesn't read a disc anymore. My advise is to send off both your drives and have them straightened out properly, the last thing ya wanna do is tank that v78 with your key on it any worse than it is now, you'll be up crap creek and getting another motherboard for sure if that happens.
Plenty of tutorials on key and drive swapping in the 360 Newbie Chat and Technical DVD-ROM Pinned sections to go thru and see most of what's involved. Since you have a v78 drive though that means getting the key off it is done 1 of 2 ways, either the drive can spin up a disc (has to to dump the FW and get the key) or the FW chip gets removed to dump it with a programmer.
You may want to look into what exactly is wrong with the Hitachi also, might just need something easy done to it to get it going again. What is wrong wit it exactly?