Always make sure that a drive is jumpered well. These are a set of pins at the back of the drive where you would see a little thing that connects two pins. By moving it according to a legend (usually very close to the jumpers), set it to MASTER, SLAVE or CS (Cable Select) depending on what the other drive is set to. Usually it would go like this:
Refer to above image to find it
1) if your drive now is MASTER, set this new drive to SLAVE
2) If SLAVE, set to MASTER and ...
3) if CS, set the other to CS as well. Hope this helps.