In PCI subsystem Setting: PEG 0 and PEG 1 set to Gen1 you may have to set that to auto i couldn't get a Linux OS to run on a 4 card rig till i set it from gen 1 to auto .
MSI Motherboards
For ALL GPUs
Under "Windows OS Configuration" -> set WINDOWS 7 INSTALLATION MODE to DISABLE and set FAST BOOT to DISABLE.
Under "Integrated Peripherals" -> set HD AUDIO CONTROLLER to DISABLE.
For RX Series GPUs (in addition to ALL GPUs section)
Advanced Mode (F7) -> Settings -> Advanced Settings -> PCI-E Subsystem Settings -> set 4G DECODE to ENABLE.
set PEG1 to "GEN 2"
set PEG2 to "GEN 2"
set PCI Latency to "96 Cycles"
For HD and R9 Series GPUs (in addition to ALL GPUs section)
Advanced Mode (F7) -> Settings -> Advanced Settings -> PCI-E Subsystem Settings -> set 4G DECODE to DISABLE.
set PEG1 to "GEN 1"
set PEG2 to "GEN 1"
set PCI Latency to "96 Cycles"
from here
http://ethosdistro.com/kb/#many-gpus