Enable Above 4g Decoding was exactly it! I had to quickly switch my boot drive from mbr to gpt (took less than a minute with mbr2gpt) and the system is posting and Windows is correctly identifying all 6 gpus. Next stop is bifurcation the two x8 slots in x4x4. Thank you for the advice, pretty amazing that the x399 (taichi) seems to be capable of running 8 gpus at x8x8x8x8x4x4x4x4…from a post-production/rendering standpoint, that makes it an absolutely killer system.
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Just curious, can you share what you’ve come up with to support the riser and extra cards?
For the x16 riser/bifurcator I’m using the AmeriRack ARC1-PELY423-C5V3 and then I’m just running two Asiahorse PCIE extenders (cheapest I could find, around $12 each). That’s working perfect on the x16 slots. I took a shot with the x8 slots (tried both) and it seems like only one gpu will show up in x4x4 mode. I suspect this may be because the PELY423 is specifically for x16 to x8x8. On their site, AmeriRack mention another model that sounds like it might be specifically for x8 signals, so I’ve emailed them about that and will give it a shot if it sounds like it will work. For a case, I’m using an open air 6 gpu case, but it’s clearly too small for all of this. If I can get the x8 slots split as well then I’ll move everything into an 8 gpu open air case.
Nice, glad you got it working!
How does one obtain these risers exactly?
Do you just have to email Ameri-Rack directly and hope they’ll sell you one?
That was the route I went. There are only a small handful of resellers that I can find and most don’t have stock. But AmeriRack’s customer service was great.
Hello atothen. Where you able to find out if AmeriRack made x8 risers to x4x4? I have a dream project of making a Dual user (Or triple since two cores are left over after two are dedicated to host Esxi) build that can be crammed into a Cerberus x.
Parts list
CPU: Threadripper 1950x 16 cores 48 lanes two 6core vm’s, 2cores for Hypervisor(KVM), Possible 3rd vm with last two cores.
Cooler: NZXT Kraken X53 240mm Front of case
Mobo: ASRock x399 Taichi
RAM: 32GB
Storage: 2x2tb sticks of NVME
Storage: 2x2tb drives of SATA
Graphic Card: 2 rtx 3070’s 
And a hd radeon 6850 sfx black
PSU: Will change but the biggest sfx psu I could find is the silverstone sfx 800 which has voltage instability.
2 Sound Cards
Note: Specific to x399 Taichi
PCIE1 (PCIe 3.0 x16 slot) is used for PCI Express x16 lane width graphics cards. 1st 3070
PCIE2 (PCIe 3.0 x16 slot) is used for PCI Express x8 lane width graphics cards. Two Sound Cards bifurcate using an x8 riser to x4x4
PCIE3 (PCIe 2.0 x1 slot) is used for PCI Express x1 lane width cards.
PCIE4 (PCIe 3.0 x16 slot) is used for PCI Express x16 lane width graphics cards. 2nd 3070
PCIE5 (PCIe 3.0 x16 slot) is used for PCI Express x8 lane width graphics cards. One Nic or an x8 riser to x4x4 for additional x4 lane card.
Does the X399M also support bifurcation? I cannot see any PCIE options in the bios 3.30.