As for Config Magic, you have to get a copy at "the usual places" (do a search for that on the boards and you'll most likely find a source.)
The new key for your new hd will be different. It's based on a few things, including your HD model. ConfigMagic generates it all for you.
Why do you even want to lock your drive? You don't need it if you aren't booting to the MS dash by default or using xbox live