I dont understand your setup, or question. If you have a standard motherboard(MB) then usually you will have two connectors for IDE cables. One IDE cable for each connector two IDE devices each cable. If you have only two devices hooked up, say a hard drive and a CDR then you could be set up with both on one connector/IDE cable and jumped as master/slave. This setup would leave one MB connector free. So you would just need a IDE cable.
With the same hardware you could also be set up with one IDE cable to each MB setup with each device jumped as stand alone/master. In that case you would just add your other drive to an open plug on one of the cables and rejumper both the drives on that cable, if needed.
If you have a standard 2 IDE connector MB, and you have 4 devices already connected, then yes you would need an IDE controller card to add another IDE device.
That new drive may be formatted, however your present drive contains the boot info and will boot. Accordingly your newly added drive may also has boot info -but will not boot. That existence of a second set of boot information on a non booting drive may cause that drives parton to become unreadable to your booted OS.
Sounds like all you need is an IDE cable? But I'm not sure, have you looked into your computer?