1) Where does it "Say" that? In a benchmark, the BIOS, Device Manager, what?
2) What model motherboard? It would be pretty funny to find that it's an old one that has 1.5 Gb/s SATA ports on a second controller.
3) Swap the SATA ports - the slow drive and one of the fast ones. If the 1.5 notation, wherever it is, moves with the drive, RMA it. If it stays with the SATA port, use another SATA port.
Let us know the results.