Your Pro Tools HD Native card should be installed in PCIe Slot 3 or 4 of Woodcrest and Cloverton MacPro computers for Optimum Performance.
Clovertown and Woodcrest Mac computers cannot support a PCIe device of speed greater than 1x in PCIe Slot 2 without PCIe
Slot 1 being lowered to 8x. (Note that Avid does not test graphics cards at less than their ideal speed of 16x.) To avoid this problem, install your Pro Tools Native card in PCIe slot 3 or slot 4 of your Clovertown or Woodcrest era Mac computer.