@pawx - Sorry I missed your post and didn't reply back in June!!
I sent you 12 or maybe 24 BIOS (not sure what all is in second set, didn't look right now), but anyway, by now you should be abel to tell me which BIOS exactly affacted the slot you want to change now, do you not know?
If you do not know still, please use a graphics card and test all 12 initial BIOS with that card in the slot you want to change now, and see which BIOS cuts the PCIE Cards lanes down to x8 or x4 etc.
I can set any bifurcation allowed/possible on any given slot, but I don't have this board so you have to tell me which BIOS setting (by way of which BIOS out of the 12) controls which slots setting.

From post #12-13 we know >> IOU1 = second PCIE slot - And this is all I can see clearly from your comments and results so far.

Post #14 does not help me any, I need full BIOS file names from what I sent you.
For post #15 results, what did you test in those slots? You said >> works in PCIEx8 - MB slot 2 << This not possible based on BIOS name, assume I set 4x4x bifurcation so x8 not possible, or the setting changed in that BIOS does not apply to this slot.
And saying "Works in slot" doesn't tell me anything, what works there, and how is it working, being bifurcated, and if so to what etc?

For clarity here, from the get-go, you need to test all 12 BIOS I sent you with a single graphics card. Test each BIOS one by one, checking each slot one by one on each of the 12 BIOS.
To see which BIOS cuts it's lane width down and in which slot, then give me results of all those tests so I can make an outline of what settings apply to which slots

Testing that way is easiest way to see the slot width being affected by BIOS in testing, or not.
Using Hyper for this is not best way, due to how it reacts with some bifurcation applied vs others, you could maybe see in HWINFO BUS area, but use only two cards and test all 12 BIOS w/ card in each slot one by one per each BIOS.

My notes on testing with Hyper II card, maybe help you with some oddities if you were not already aware
[REQUEST] Bifurcation Asus x99-a II - 5820k (2) (See post #16)

@Lost_N_BIOS

Thanks for the reply.

You have only sent me 5 different BIOS options, which were:
X79UD5NVMEIOU14x4x
X79UD5NVMEIOU24x4x8x
X79UD5NVMEIOU28x4x4x
X79UD5NVMEIOU34x4x8x
X79UD5NVMEIOU38x4x4x

And these were enough to figure out which IOU is which PCIE lane:
IOU1 = PCIE_2 (8x electrical)
IOU2 = PCIE_1 (full 16x)
IOU3 = PCIE_3 (full 16x)

I did further testing with a GPU. Here are the results:
IOU14x4x
PCIE_1 16X
PCIE_2 4X (both NVME SSD show up in windows - bifurcation works)
PCIE_3 16x

IOU24x4x8x
PCIE_1 8x (both NVME SSD show up in windows - bifurcation works)
PCIE_2 8x
PCIE_3 16x

IOU28x4x4x
PCIE_1 4x (NVME SSD will not boot)
PCIE_2 8x
PCIE_3 16x

IOU34x4x8x
PCIE_1 16x
PCIE_2 8x
PCIE_3 16x

IOU38x4x4x
PCIE_1 16x
PCIE_2 8x
PCIE_3 16x

It appears the IOU3 BIOS does nothing to the third slot. Do you have any idea why this might be so?

I did read that some motherboards might behave differently with the ASUS HYPER m.2 card - ex. only 1st and last slot on the card might work with bifurcation. But my m.2 carrier card only has 2 slots available so I can`t test this.

Any ideas for further changes to the BIOS?

Thanks :)

edit: As luck might have it, PCIE_3 slot is the one I need bifurcation to work on. Alternative solution might be to try and squeeze a flex ribbon PCIE riser under the fat GPU, but I`m not sure it would fit.

@Lost_N_BIOS

Hey. I have another question.

I`ve downloaded AMIBCP 4.55 and opened the BIOS file. I see where the hidden menu is and where the PCIE lane "width" is assigned (16x, 8x8x, etc.) Is this the only change I need to make to have bifrucation or are there some other additional steps to perform?

Greetings and happy holidays.

Hello.

For anyone wondering.

After some testing and researching it appears that this motherboard "Gigabyte X79 UD5" only supports bifurcation on first two PCIE lanes.

I`ve solved my problem using a PCIE riser and by mounting the GPU vertically.

Greetings.

Has a final release been made for this Bios?

@Lost_N_BIOS
Hello
Could you please share bios versions with bifurcation enabled?
X79UD5NVMEIOU14x4x4x4x
X79UD5NVMEIOU24x4x4x4x
X79UD5NVMEIOU34x4x4x4x