Well I didn't enabled anything in bios before windows setup but I've made bootable usb with Rufus, GPT partitioning for UEFI systems, so I assumed that it'll for sure be UEFI.
you need to choose Windows 8/8.1 in the BIOS (enable UEFI) instead of Windows 7 and (you wont be happy) you need to reinstall Windows 10.
I had a similar problem, my rig was fine with 4 cards but not with 5 or 6 until I reinstall Windows 10 after selecting Windows 8/8.1 instead of Windows 7 in the BIOS.
Of course if it was already set to Windows 8/8.1 then your problem is something else.