Do you know if you've got a rev1 or rev2 B&W, if you've got a rev1 then the HD controllers aren't very stable and it could be it isn't compatible with your new drive.
There's a bit more info on the lowendmac site
http://www.lowendmac.com/ppc/blue-white-power-mac-g3.html
the rev2 have an IDE controller marked 402, which I think is down near the IDE plug, not 100% as our rev2 is a plane journey away, few years since I worked on it.