I have a Dell PowerEdge 600SC running Win 2000 Server. Currently there are two hard drives controlled by an ATA-100 Host bus Raid adaptor for Poweredge SC. (#9K646) in a Raid 1 (Mirroring) configuration. I want to leave the raid configuration intact but add a third hard drive as backup storage.

Do I connect the IDE cable to one of the available slots on the host bus Raid adaptor? Do I connect it to the available IDE slot on the MOBO? Either way, how do I configure the bios?

Your boot drive is WHICH drive?
It isn't going to matter much which controller you use for the third drive. It usually doesn't matter for IDE devices as the controller you connect it to will probably do the config. automatically. Just be sure that you set the drives jumpers correctly before connecting and powering up.
Personally I'd use up the system board's headers first, the Raid controller second.

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