Also, about several days ago I've replaced flat ribbon powered riseres on my oldest rig alive. I'ts been running with two PSUs non stop for about 4 years with no issues ; ) Except some risers burned cause of bad solder.

One PSU(small one) is powering motherboard and stuff and 1 riser and the same GPU which is using that riser. The other is powering 5 other risers and GPUs.
Rig used to run 6950/7950/290 cards.

The PSUs are different. Also one of them(small one) had capacitors gone bad and rig started to freeze. After capacitors replcaement all is good till now ; )

In this case - one PSU is powering all the risers and one PSU powering PCIe power connectors - there may be current flow through the GPU from one PSU to another. In the case of one PSU is on and another is off - the one which is off will/may blow up because it will receive voltage on it's output. And the PSU which is on may/will blow up because who knows how many W the off PSU will consume using A through it's output ; )
And the GPU won't be happy at all. Prly also will burn.

However, ppl used to do it because of unpowered/un-cut risers - for example flat ribbon without +12v additional power. In that case riser -
Nowadays, mostly ppl use USB risers, which provide all the voltages to GPU from it's external power connector. So they don't use PCIe bus voltages.

I think this was the issue.
How exactly did you hook up second PSU?

When using dual-PSU, you must be sure, that no current flows between both of them through the GPU.
For example powering riser with one PSU and the GPU with another through PCIe power.
Also, PSU grounds must be shorted.
And both PSUs must be on(or off) at same same time always. If not, one PSU will feed some power into the output of the another PSU.. and it will/may blow up.

Also, maybe you just overloaded PSU and it dropped +12v to.. say 10v or ripple have risen and killed the cards. Or both.

[WTB] RX 480, EU

The seller refused to accept BTC via escrow and PayPal..

Maybe Windows Server 2016? It looks like Win10 tho, but has all the crap disabled by default.
Or Server Core 2016 ; ) It does not have any UI at all. I used to run mining rigs on Server Core 2008.. painfull at first ; )

Do they actually help AMD cards?
I thought that's for nVidia..

Win10/2016

Plug'em all! ; ) Run the mod app.

Tho, there may be issues with integrated GPU being primary.
You may be required to use the dummy or disable integrated GPU or try Force monitor

It won't. Or at least it shouldn't. I've seen Win7 rig with 4x7990.

But prly FirePro only, or whatever they are called today.

There is no way(known to me) to run 6x480(or 470 dunno) on Win7/8/8.1.
I forgot exactly which 6xGPUs don't work, 480 or 470.

I'm running 6x 290/390 fine on 2k8(w7 server). With any(looks like) driver version.

On 2k16(w10 server).. since Aero(not Aero per ce but d3d11 desktop rendering) can't be disabled(imo), OS UI consumes more GPU. So a bit more vCore is required. At least that's what I've noticed on my rigs.

Yes, read the thread from the start ; )
However it depends on the cards, f.e. 8x290 works on Win10 out-of-the-box.

Afaik you need special driver for BIOS-modded 470 cards.

I've compared inf from this driver to 16.10.1(which I could not make to work with 6x470)

They differ in those three settings under Polaris10(RX 480, RX 470) config

Wtf is this, I've no idea ; )

Next step would be - add those to latest driver, including all changes done by 6xGPU mod and see what happens on 6x470/480 rig.

Where did you get 16.10.2 driver, from AMD site?

Disable built-in GPU in BIOS, set PCIe to 1.1 if possible, plug GPU directly in the motherboard.

I keep reading this 'tip' for years..
However in all my experience, dunno maybe ~100 rigs, I've never seen a system which could not install driver for all the cards plugged at the same time. Tho I did see something like 'timeout exceeded', happens on very slow CPU/HDD rigs because 6 factorial driver installs takes a lot of time and Win thinks device installation stalled, all cards are working after reboot tho.

However, plugging one card at a time deffo can help eliminate 'bad' cards. Tho I personally have separate test rig, not a rig per-se just a mobo+cpu+ram+ssd which sits on top of the mobo cardboard box ; )

Disable built-in GPU.
Set PCIe to 1.1 if possible.
Before installation of any drivers install Intel chipset driver.