I did a screen replacement recently using this guide and the disassembly/assembly was fine - very good instructions. But when I turned on the machine there was a very fine flickering, snow-like interference, especially noticeable on black backgrounds. I assumed it was a faulty replacement screen and continued to use it. Recently, I had to change out the battery so I dissasembled the screen again to see if reconnecting it would fix the problem. Now it is perfect. I think the problem was that the left-most (shortest) airport cable connector was touching the black screw next to it on the board and shorting out. Be careful to position this connector well away from that screw.
When you replace the new drive, and before you close up the case again, make sure that the ribbon connector is pushed firmly onto the correct pins on the HD. Check that the jumpers on the drive (if it has any set) are set to Master. Also make sure that the other end of the ribbon connector is pushed well onto the socket on the main circuit board. You may find that the drive isn't seen if the connections aren't correct. Best to check before you put all those screws back in!