Can't get DMA6 working on ASROCK X58 Extreme Mobo

I have an ASRock X58 Extreme mobo with a Seagate 160 Gb SATA-II HDD as the boot drive but even though the BIOS tells me that it is running in DMA-6 mode when I view the settings in the control panel under Windows-7 (64-bit) it tells me that the drive is only running in DMA-5 mode and the HDD benchmark result supports that finding. Is there a way of forcing it to go to DMA-6 ??