The HF engineer in me cries "OMG THAT CABLE LENGTH!". Do you have somewhat shorter USB 3 spec cables to try this with? Maybe some of the nVidia cards are having trouble with signals getting distorted on these long cables.
Christian
Those risers aren't actually using the USB protocol - they're really just simple extension cables with USB ends on them.
the RF engineer in me knows that. The cable length introduces delays, attenuation and nonlinearities that may be well out of spec for the PCIe bus.
Still doesn't explain why the first 4 cards are legit, the 5th is half ass, and the 6th is non existent. They are all recognized, powered on, and spinning. I'm writing something to see if I can send some work to the 6th card via cuda.