I recently purchased an ASROCK Exreme 6 Z77 and noticed there is an SLI/XFire 4-PIN Power connector on the motherboard. The manual says to "connect it with a hard disk power connector when two graphics cards are plugged to this motherboard." I am not currently using it and both video cards are detected and working in XFire. I haven't seen where anyone else is using this.
Thats a Molex Connector, I dont know about plugging the "Hard Disk Connector" but I can damn well say it is Molex. Even Asus provides such a feature on their ROG motherboards and they clearly say to connect it if using SLI or Xfire.
If you're using 2 cards and you say you are, then I suggest you use it or else those cards will burn your motherboard. Connect the Molex pin with any Molex cable from the Power Supply.