Sorry alan, but vette is correct.
None of the current DNS-NAS devices will provide a SATA 3.0 interface because the hardware was designed for SATA 2.0 at the time. However, if you were to plug in a hard drive that supports SATA 3.0, it will simply step down to SATA 2.0 compatibility when communicating to the NAS device.
It's a simple case of supporting the “lowest common denominator” when dealing with cross-compatibility standards such as SATA.