You need to set the new DVD drive as "slave", which basically involves bridging a pin on the DVD drive's data/power connector to force this. It is currently set to cable select which older Apple notebooks don't like working with; they need a designated master + slave drives set.

Some info on the mod, to get you started:

http://www.applefritter.com/node/23468

... I did mine with condictive ink.

JB