@Darwinn
If you want those to work you need full 4x4x4x4 bifurcation, TR4 boards have this and RAID support. I recommend you do some research and/or start your own topic.

thanks for your efforts put into this project.
i have a P9X79 {the standard version, 2blue 1white PCIEx16 slots}, currently on the latest official bios (4801).
when i try to flash your bifurcation modded bios (from your forum signature link "Asus P9X79-4701-Mod-NVME-SA-Switch" which i assume is for this exact mobo) with the EZ FLASH 2 utility inside bios it says "Selected file is not a UEFI BIOS".
I tried putting the official 4701 bios file on the usb drive and it accepts that one (so the error shouldnt be related to the flash drive or its formatting (fat32, tried different older drives and different formatting with win/rufus too).
Both names were P9X79.CAP. However i didnt proceed with the official 4701 version downgrade yet, as well as didnt try to use the bios flashback function with your modded file because im worried about creating more trouble or bricking until i get some affirmation.
Do i need to flash the official 4701 first?
Do i need to use the bios flashback function to flash your bios because ez flash maybe recognizes it has been modified?
Does the bios flashback still work even if previous flashing process critically fails (such as powerloss mid-flash?)

This files mods are for using the Asus USB FBF feature, never EZ or any other method, because as u saw it, Asus security features do not allow modified bios files to be used.
USB FBF will always works even if the mod prevents the boot.
U should update to the oficial 4701 (EZ or FBF) then apply the 4701 mod (FBF ONLY), if later the oficial 4801 is applied u loose the mods off course.

@msdq9 Yes bios flashback is required. The BIOS in my sig is indeed for "P9X79" I recommend you reset default options before and after you do a downgrade. You should not need to flash the stock 4701 first.
Also bricked BIOS should still flash with bios flashback flashing is then done by an independent system. Most here would recommend getting a hardware BIOS flasher just in case, tho you could do that when all else fails.

@JackSted
Thanks for the excellent tutorial.

I was able to apply your guide and modded ASUS Rampage IV Formula BIOS (enabled System Agent + PCI Configuration menu).
I tested the functionality with this bifurcation board -- https://riser.maxcloudon.com/en/bifurcat...to-4x4-set.html and several GPU-s
I can confirm that it's impossible to use x4x4x4x4 config even in this case, I can only use option with x8 + x8 and 1-st and 4-th outputs of the bifurcation board work (though, at x4 speed -- but this is limitation of the bifurcation card, need to order another variant)

However, it looks like I'm running out of PCI resources for Win10 -- USB riser in slot 4 (the one at the bottom) stopped working and there are errors in Device Manager.
I tried to use "Above 4G decoding" option in BIOS -- but this leads to no output from my GTX 1080ti to monitor, and I had to clear CMOS.

Did anyone tried this option? Are there any other workarounds?

@asic
Sorry for the late reply,
Not knowing what or how many your connecting to you PCIE I recommend either buying a ~300 EUR+ card that does PLX/PFX/PEX lane switching (optional M.2 Key M to PCIe x4 NVMe Adapter cables); or switch platforms to something like amd tr4 threadripper, or x299(cannot do hardware NVME raid).
I have not tried the Above 4G setting. As for the other unlocked BIOS settings it is quite a mine field.

@JackSted
Thanks for your reply anyway.
Could you recommend anything except boards shipped by c-payne.com? 350 EUR for the board, +re-driver cards and SAS cables to get the config out of the pc case... Well, it's a serious investment ))

Other observations: I can bifurcate x16 slot (third from cpu socket) and it works more or less ok (except for the Tesla K40 that blue-screening periodically in the bifurcated slot but works ok via USB riser in PCI 2.0 x1 slot). However, when I'm trying to bifurcate x8 slot (to the pair of the x4 links) Win 10 shows unrecognized device for the second x4 link and can't install drivers for anything: tested with several GPU-s, WiFi card, NVME with adapter. Did anyone tried the x8 slot bifurcation with other type of re-driver cards than I'm using - like dual M.2 cards?

I see in the bios the 8x slot has the x4x4 option. Anyone try that slot to use 2 NVMe drives ?

I am thinking about a card like this

https://www.amazon.com/gp/product/B09F31ZXKQ/ref=ox_sc_saved_title_1?smid=A1YVSYXQX2ABEU&psc=1

thanks