Yes, powered risers dont get power from the motherboard so without power from a molex the cards wont work, or atleast shouldnt work well
Not always true.
If it's a simple riser (not USB), like that :
https://www.hashratestore.com/wp-content/uploads/2013/04/powered-pcie-1x-16x-16x-riser-cable-molex-5-400x260.jpgThere is 90% chance that the riser is uncut, It means that the power supply with molex is connected on the wire but the power pins are still connected between the 16x female slot and the 1x male slot.
So a powered riser, will actually works without the molex plugged, but It will draw the power from the mobo (I run some cards like that, and you can check it with a voltmeter). If you plug the molex, the main power source become the molex.
That's why on this kind of riser, it's highly not recommend to do a dual PSU with PSU 1 on the molex and PSU 2 on the mobo, because they will share their "ground" and it can happen some electrical problem. (from what I read on some topics when I was hesitating between 1000W PSU or 2x600W when mining on scrypt
)
Thanks,
Yeh i get what you mean, that's why i said "atleast shouldnt work well" because with his 3 cards on non powered, that's a lot of power to suck from the mobo, especially with x16 to x16 risers.
I think the cards may have been running into pci-e conflicts and interrupt problems because it was populating so many pci-e lanes.
Obviously using x1 to x16 risers drops the number of lanes and stops the conflicts
