I have added an extra drive to my tower using the second plug from the existing drive but there is a smaller plug ( blue Wire) that comes from one of the boards. Next to that connection there is a secong identical connection so got a second Blue wire and ran that from the board to the extra drive. The only drive that shows up under my computer is the original drive. The second drive can show up if i put the original blue wire in to that one instead without removing the other plug.
I was doing a job with a bunch on Dells the other week:
I would imagine the blue wires you are talking about are SATA cables and connectors?
Connect the drive, go into your bios (might be F2? It should tell you somewhere on the logo screen immediately after power on what key it is)...you have to enable the second SATA port - it might be under interfaces or drives (can't remember now).