CDROM DRIVE
A CDROM drive is an ATAPI device. From an early IDE document:
"Beware that although ATAPI devices plug into the IDE interface, they differ considerably from an IDE hard disk. Caching controllers and other intelligent interfaces will not work unless they're ATAPI aware."
Maybe that's the scenario you're in - an early IDE controller.
MULTIPLE HARD DRIVES ON ONE IDE CABLE
In the very early days of IDE, getting drives to master/slave was painful. Sometimes to get a combination going, one had to swap the drives - master became slave, slave became master. Generally, if you stuck to drives from the manufacturer, you'd be right, but even some Seagate combinations failed. If I remember correctly, it worked every time I used drives of the same maker/model.
That was back in the days of drives under 100MB is size. The situation improved since then, but it can still happen. I've still got a Fujitsu 6GB drive that would not master/slave with one of my similar Maxtor drives.