It's a known problem with some Z77 chipset motherboards.
If there is a Gen2 choice instead of Gen1 for the PCIe 3.0 slot then choose it.
The only problem with the PCIe 2.0 slot is that it only has 4 PCIe lanes.
You can always return the motherboard and get a better one that doesn't suffer from that problem.
Is this a problem with all ASrock z77 mobos? Or varies from board to board?
Also, there's no Gen2 option, so should I move it to the 2.0 slot? I'm not sure what would actually be faster.
The ASrock people are trying to tell me the BIOS for the 670 is at fault and needs to be updated, which I've contacted Galaxy just to make sure, but I'm thinking Asrock is deflecting and trying to convince me I don't need to return it