that sounds like the 'old' standard molex connector used for drives and fans so its possibly for the front fan. Plug it into the simular version connector on the power supply if you have one. My CM case's front fan was a 4 pin plugged into a molex adapter so I just unplugged the adapter and then plugged the 4 pin fan connector to the front
Of course it could also be power for the top mount external drive dock. Again plug it into a molex connector from the power supply.
If you only have sata power available on your power supply and it did not come with an adapter - you can buy an adapter such as: http://www.newegg.com/Product/Product.aspx?Item=N82E16812816038&Tpk=sata%20to%20molex