I replaced the cd-rom in an iMac DV, and now the computer will only boot from the CD if the HDD and CD are both connected. When I disconnect the CD, it will boot from the internal HDD just fine.
I tried different jumper settings and whatnot, but nothing seems to work.
Do you think the little power board on the CD-rom might be the culprite? I have no clue at this point.
Any ideas of other things I should try? Or is this iMac going to be forced to use an external HDD forever? 
Does the system "see" the hard drive when both are connected, and just not boot from it, or does having the CD-ROM present render the HD invisible?
Boot an OS CD and use the Disk Utility to see what's present on the IDE bus. If I recall correctly, the "slot load" iMacs share one bus between the HD an CD, so you should see two seperate devices. If you only see one, the CD-ROM drive, see if it's in the "master" position. If so, that's probably your problem.
(Does your replacement drive have any drive-select jumpers or switches on it? If so, they could be overriding the "proper" setting. Which I imagine should be "cable select", provided by the little adapter board.)
Also, for sanity's sake, make sure your HD isn't set slave. That'd also cause a bit of a problem.
--Peace
If it boots from CD, it doesn't see the hard drive at all. If I disconnect the CD, it boots from the HDD. It just won't see both at once.
I tried all the different jumper settings, they have no effect.
My apple tech friend suggests I try a different optical drive. He says he has encountered this before when fo no reason a cd-rom won't let you see anything else connected to it.
I should have access to a slot-load DVD-rom next week. I'll see if that works.
Some optical drive have a small slider switch that allows you to select Master Slave or Cable Select. But some are fixed at Slave or Master from factor. From experience, Panasonics/Matsushita, older LG's like to solder the jumper on permanently.