IDE controllers are able to connect two devices at a time with the option to use a Master/Slave connection protocol. On all IDE drives, there are a set of pins on the back with a jumper connecting two pins to enable that switch. If your old IDE hard drive was set to master and the DVD to slave, when you removed the hard drive without changing the DVD's jumper to "CS" (for cable select) or Master/Single.
In addition, some older machines have BIOS that do not auto detect drive types or lack of. So the BIOS might have to be explicitly set for just the DVD (as master) and no device as the slave. (disabled).
Lastly, be sure to use the end socket on the IDE cable (usually marked Master), not the middle socket, with the controller end on the motherboard (check that its not reversed).