If that cable has another branching off of it, then it's not SATA. SATA works on the concept of one device on one cable. Sounds like it ATA. If it is then check the master<> slave jumpers. It sounds like to me it's on the same cable as your optical drive and the jumper on the CD and the hard drive are set the same. Make the cd drive master and the hard drive slave. Then it should see it. Remember you have to format the drive before you can use it.
Oops! my bad. I just looked over your posts again and you say it comes off the other HD. Same difference though. Set the jumpers. The one in there now should be master and the new one slave.