You probably didn't set the jumper at the rear of case properly. The default setting is usually master (MA), and if the replaced drive was configured as slave, you would have two drives incorrectly configured as master. The result would be that neither would be recognized due to this error.
Check to see if the DVD-ROM is connected to the outermost end of the IDE cable. If so, that is where the master device should be.
I would remove the DVD-ROM and set its rear jumper to slave (SL) and connect it to the inner IDE connection. I would confirm that the DVD-RW is set to master and connect it to the end of the IDE cable.
Hopefully, that should get you going.