I'm not 100% sure on this (someone *please* correct me if I am wrong!) but I believe only one device at a time can be active on a given channel, primary or secondary. The result is that when both the hard drive and CD-ROM drive are on the same channel either the hard drive can be active, or the CD-ROM drive, but not both. Thus, it is usually better to place the CD-ROM drive on the secondary channel; they can then operate independently.

I would also be interested in any online info on this subject. Anybody find anything?