H81 Pro BTC and powered USB risers

[beewerks]

Hey guys, I've got an Asrock H81 Pro BTC and am trying to run 4 R9 280x all powered off of a Silverstone 1500 watt PSU.  I'm using powered USB 3.0 x1 to x16 risers.  I can get one card recognized, but none of the other cards work.  This is with the risers being powered and the onboard 2x molex connectors disconnected.  I unplugged all of the power to the risers, and plugged in both molex connectors on the mobo.  When doing this I found that that the gpu fans won't spin up at all.  Do I need to have the risers powered as well as the 2x molex's plugged in on the motherboard?  Any help would be awesome!

[Bomtrack]

Did you get this figured out? I'm running the same board with 6 7970 and win 8.1.  I don't use powered risers now, I did at first then i found out u you don't need to, just use the molex. I have the 6 and 8 pin power connectors on the gpus connected or 8 and 8.

[byt411]

As far as I know, you don't need powered risers.
The guy in this video insists to not do so.
https://www.youtube.com/watch?v=t3iup26oUN4

[wijnand1]

i have had the same problems connecting 4x 290x on the mobo. after a lot of hassle i got them all 4 working with powered risers and the molex in mobo not used.

On another rig i used the molex on mobo to power and didnt use any powered risers. this gave far less trouble and is working stable ever since. The trick is not to use the 16x 16x riser at all. Just use 1x 16x on all gpu and no powered risers