Mondial When you fit a new drive, if it is the only DVD/CD drive in the machine the jumper which is situated on the back of the unit should be set to 'Master' it's a little plastic coated connector which bridges any 2 of 3 or 4 sets of double brass pins, most units will have an indication of wich pins represent Master, Slave or Cable select (CS).
The Grey ribbon which usually has a red line down one side, is pushed onto the pins with the red line closest to the power connector. It's some times possible to put this ribbon on the wrong way so just make sure the red line is next to the power connectors.
If you have two DVD/CD drives on one grey ribbon one must be set to Master and the other to Slave, the slave drive connects to the middle connector of the ribbon and the red line rule applies here as well.
Hope that helps, it's an easy job to do once you 've done once.