A lot of replies, thank you guys.
Yeah, but Corsair HX series can be switched to both and it comes set to Multiple.
So my toughts are the mobo...
I haven't changed anything to the setup on the PSU, it's as it came out of the box so I'm not sure if it's multi or single. The HXi required the Link connection enabled/installed to allow you to change this and I haven't done anything for it, so it's not easy to try. Also, you mention you think it's the mobo. I've seen and read here of setups on the Gaming M5 with 7 GPUs, how can it be the mobo? And let's say the first mobo was the case.. 2 more? So 3 in total faulty? Lastly, why would single vs multi rail make such a dramatic difference and cause things to fry?
Did the system work with the 6 GPUs prior to adding the 7th?
Is there any chance that ANY of the cables connected to the PSU are not Corsair PSU cables even if they look the same?
Use a multi meter and check the output at the cable end and see everyone has the voltage and polarity that it should.
Yes, it did and it currently does with 6 GPUs. No, all the cables are Corsair cables, they were part of the Corsair package plus the extra ones I bought were from Amazon and they are from Corsair as well (I bought two extra kits to have spare cables to use), I mean they came in a corsair box and bag, I don't think they are counterfeit. I will use a multi meter to measure the output, that was my idea after all this shitfest as well, but I was trying to poke people's brains in case they had a similar experience or any other ideas.
At this point, I have an AXi 1200 and 7 other GPUs waiting to be assembled into another rig, but I'm afraid to try, before I understand what happened. Even if I was trying to draw more power than what the PSU can handle, it should just have shut it down, not fry everything. On the other had, looking at how much power 6 cards are drawing now at the wall, even under-volted, it's about 1080 to 1120.. so I have doubts if it can handle the 7th card now, I was expecting the system to be around 920-950 with 6 cards when I did my calculations. I might have to get a secondary PSU and link them together.