I've been running a few mining rigs, which are basically just motherboards crammed with cards.

Due to heat problems, I've been looking to get some risers - and they finally arrived. I got 100 cm flexible USB cable risers, with SATA power. They look impressive 3 filter caps and a VRM on the riser board.

Anyway, the first thing I found was that no cards would be detected on the risers until I set the mobo BIOS to PCI-e v1.0.

So far, so good. So, one at a time, I transferred cards from mobo onto riser cables, and after a few false starts, I had 2 cards running on risers, and 1 on the mobo.

That's when I start to get adventurous, and connect a 4th card to a 1x PCI-e slot, and power it via a 2nd PSU. All the connections are double checked. The riser is in the slot the correct way round, etc.

Link the PSUs with a dual PSU adaptor, and power up.

*Sizzle* *Pop* *Crackle* <Burning smell> Cue sparks and smoke rising from the 1x PCI-e slot. The mobo is totally dead.

I recheck all the connections, and they are definitely correct.

Thinking I made a mistake, I decide to have another go.
This time, I connect directly a 1x PCI-e slot with a 4th card, after triple checking everything.

*Sizzle* *Crackle* Sparks. Another motherboard now dead.

I don't get it. I've built dozens of PCs in the past, and built a heap of mining rigs, albeit without risers. I've done dual PSU before.

What could have happened? What was cause sparks and sizzling and burn out a mobo. I've looked at the PCI-e riser design, and the power pins are not connected on the riser, so I don't see how a power surge or something could have come from the 2nd PSU back feeding the PCI-e slot.

Any ideas? Could it be a bad riser card? I've run a multimeter over it, and there don't appear to be any shorts or anything. I'm completely baffled.