So you mean to say you already have two devices both connected to both the IDE port the motherboard and now you have purchased additional hard disk for your system. The only thing is yoy get two connector along with the single data IDE cable so buy a IDE cable which should have two connectors on the one side and single on the other end and connect the single one in the IDE port and the other connectors to both the hard drive and check whether the jumper setting are correct as it will be shown on your hard drive one main hard drive should be kept as master and the other as slave. It doesn't matter on a secondary drive which drive is master and which is slave. If you put the new drive with your existing hard drive make sure that the new drive is set to slave since the original will be master.