No, the B75 chipset can support two SATA III ports. There are 2 SATA III (bottom 2) and 2 SATA II (top 2) ports on the i660.
I had the same problem with a Seagate Baracuda 1 TB SATA second internal drive. Only showed 3 GB speeds in a variety of tests. Many complaints on-line about this issue with this drive. I switched the 2 TB Seagate SATA III boot drive to the second channel and still got 6 GB speed. Using the non-OEM cable made no difference.
In reality, SATA III is not important with most hard drives, as transfer rates of 3 GB/sec are not usually possible. I get 1.2 GB/sec speeds transfer speeds, much lower than SATA II. With SSDs however, there may be a difference.