Do you have your Operating System on that SSD? If that is that is the case and you installed it using an image (clone) then you might want to test reinstalling the Operating System while the SSD is connected to the SATA 6Gb/s port.
Also if your board has more than one SATA 6Gb/s controller (i.e Marvell and Intel) test the SSD on the other controller.
Make sure too that you have the latest BIOS and run the latest SATA controller and chipset drivers to rule that part out.
Next friday I will get my 240 GB SSD Series 520, the motherboard has 3 SATA 6GB/s ports, once installed how can I verify that the SSD is running at 6Gb/s and to do some performance tests to ensure everything is fine, I will return to my country and I want to be safe. My new PC came with Win 7 in a slow 5400rpm HD, I will do a dual boot, installing from the ISO image PCLinuxOS in the 240DG SSD.
I appreciate this information since I do not have the SSD yet and I want to be prepared because the SSD will be installed by the vendor, and I will have little time to install Linux and test the SSD.
Check with ATTO.
If your max speed is over 500 MB/s your SATA3 port is working fine. If it is around 250MB/s then you are on SATA2.