The majority of all CD and DVD drives for computer systems use the IDE interface. This interface can have two devices on a single cable. Each device on the cable must be placed into the appropriate mode for the cable. One drive is listed as the master and the other secondary drive is listed as a slave. This setting is generally handled by one or more jumpers on the back of the drive. Consult the documentation or diagrams on the drive for the location and settings for the drive.
If the CD/DVD drive is going to be installed on an existing cable, the drive needs to be set into the Slave mode. If the drive is going to reside on its own IDE cable alone, the drive should be set to the Master mode.