no no no! haha, do not flash your drive!!! you do not have to do this if they are the same model!
K, heres what you do. Get your OLD drive, unscrew and remove the metal casing.
On the bottom youll see the rectangle computer chip. Its held in place by two plastic black clips.
Push one of the clips over so you can lift that side up (do this slowly as not to rip any cables just in case), once that side is lifted up the other side should come right off.
Now the cables should all come off very easy, just slowly pull them out of the board.
There is one that is taped to the board, just slowly pull that one off the board, dont worry its just taped so its not gonna hurt it.
Once you have it completely removed, Sit it to the side somewhere, and take your NEW drive and do the same thing, once you have it off, Sit your new drives chip in a safe storage spot, OR put it on the OLD drive.
Now take the chip you took off of your OLD drive and put it on your NEW drive, Plug all the cables in where they go, Clip it onto the black clips and you are SET! Now your 360 will read the new drive as if it was the old one because its reading the old ones firmware!
This is the PAIN FREE, FAST, and EASY way, takes no skill, no modding, nothing. Takes all of 5 minutes to do. Its how i replaced my drive a few months ago. Need any more info or a walk through in real time lemme know, i can chat with ya on aim and guide ya.