You need to tell us which boar you are using.
However in Bios there is settings related to setting which preferred GPU should be used. PCIE connected one or IGPU. Make sure it is set to PCIE. once you have it up running then turn off the IGPU power completely in the bios also. It is thus not enough that the bios detect that there is a PCIE GPU the bios will still want to turn on the IGPU.
Regards,
It started with a VIC-20... and now it is. Maximus Formula VI Intel 4770K 4.6GHz ASUS GTX780Ti 3GB Corsair Platinum Dominator 16GB 2400MHz Raid Express 240GB Samsung EVO 512 GB SSD Corsair 900D EK watercooling Corsair RM1000 ASUS PB278Q