Just because there is a socket does not necessarily mean it is active. You would need to check the manual to see how many SATA devices can be connected. The spare socket (if active) would be able to run a SATA HDD in conjunction with that spare SATA power connector. It depends which version of SATA your motherboard supports, 1.5 or 3 which dictates the theoretical speed of the devices. There are no master/slave issues with SATA.