When you are adding the DVD drive is it on the same cable as the HDD?
If so you might have them both set to master, there isa little set off pins on the back or you DVD drvie , that have a little Plastic sheet over too off them, and it should be Labled M S CS , you will want to put the platic sheet over the one that is Labled S
M = Master
S = Slave
CS = Cable Select