I managed to get it working after some research. It looks like Western Digital drives can't be used with the jumper in the neutral position. They must be assigned as either master or slave. I think I have it in a usable position but there's a small glitch. Whenever I have an OS disc in the CD drive it automatically boots from that instead of the hard drive. I thought you had to hold C to get a CD boot? Maybe I slipped the jumper over the wrong pins?

I'll be trying my g4 450 upgrade in it next.