Oh ok, I didn't realize that the Xbox wasn't modified. Well true, it won't boot. Hmmm, if that drive is locked, it needs to be unlocked before you can put it into another box. You can attempt to use that key to unlock it with your PC using HDUNLOCK.EXE after booting from a boot floppy, but...the method you used to obtain the key is most likely unreliable and you will most likely not succeed in unlocking the drive.
The only way I can think of that would possibly save you is if you have a backup of the eeprom.bin from the unmodded Xbox. If you do, you could flash the eeprom on your modded Xbox to match the unmodded one, and then it would accept the drive, allowing you to boot to a CDRW etc, and then you could recover the drive's data and put it back in the unmodded box.
Basically, you're in a pretty unrecoverable situation. I wish you luck.