Thanks for the suggestions. BIOS is updated to the latest.  I've already tried looking for the GEN settings in the BIOS but found nothing relevant.

By shorting, I mean this:

http://en.community.dell.com/support-forums/servers/f/956/t/19373337

No issues with the MB afterwards, except still no detection.

Will try running DDU again, and then installing the GPUs one by one via the risers.

Hi,

I have an A88X-PRO AMD MB, running Win 10 and trying to install 4 RX580 4GB GBPs.

Installing 2 GPUs onto the MB directly work fine (PCIE_16_1, PCIE16_3), however any that are connected by to the board via 16-1 PCIE USB risers do not work.  I've tried sourcing risers from 2 suppliers (12 risers in total), as I had heard that some risers are defective, but the GPUs are still not recognised by the MB.  They light up, but the fans don't spin and nothing appears in Device Manager.  Also tried the trick of shorting one of the PCIE1 connectors on the MB, but still no luck.

Any help / suggestions would be most welcome!

Thanks.