Sounds like ya bricked it to me. You'll need to remove the FW IC and reprogram it with an external programmer. That whole process involves 3 kinda involved steps, removing the epoxy around the SST chip, removing the chip itself, then having a programmer to unbrick it. If ya have all of the necessary tools and skill to do it then ya probably wouldn't have posted, so I recommend sending the drive off to someone that can do that kind of thing correctly.
You also have the option of replacing the entire drive also, since you do have a backup of the FW and key. Just have to dump the new drives FW, inject that key into it and then spoof it to report it's really a v78 if it's a different model or version.
There are guides in the 360 Newbie Chat forums pinned section on how to install a new drive, key injectinf, spoofing etc. As for removing the SST chip and unbricking it with a programmer, there aren't any guides that I know of, and that's something that really needs to be done by someone with the correct tools and experience anyway, especially if you're going to attempt to salvage the drive, ya don't wanna go at that process unless ya know what you're doing or you'll be replacing the drive for sure.