I don't get the 6-pin splitted to feed risers.
The 6-pin or 8-pin cables that you plug into the GPUs only carry 12V but the riser also needs some of the 5V feed included in SATA/molex.
Edit: nevermind I see there are risers that only take 6-pins and stepping the 12V down.
Whatever the case might be, I think either the PSU is faulty or something is very wrong since I never heard people also frying stuff like CPU/RAM/disks with the motherboard.
I'd be curious to see some pictures.
Yeah, I was surprised by this as well. I've been building PCs for a long time, for an internet cafe chain back in Europe (about 5-6 per day + fixing and diagnosing on some more), I never had such a case. I was annoyed by the whole thing.
I do suspect the PSU as well, or at least the specific output port of the PSU where that cable plugs in but at the same time I have trust in Corsair not to screw this up or at least catch it in their QA.
Is there anything specific you want to check at the pictures or just the whole setup? Currently is with the 6 GPU.