I have:
- 1x ASUS H270-Plus mobo with 6 PCIE slots (2*16x, 4*1x)
- 6x XFX RX 480 GPUs
- 6x Ver007 Powered USB Risers
- 1x EVGA 1300 PSU
- 8GB of Ram and an Intel Pentium CPU

I'm powering the USB Risers

I can boot Windows and SimpleMiningOS with 3GPUs plugged in, but not 4. If I plug in a 4th GPU, SimpleMiningOS boots, but the screen stays black, and hitting any key reboots the machine. Windows won't boot at all.

I power each USB Riser using molex connectors hooked up to the PSU.

I've tried setting each PCI-E slot to Gen1 (and every other option: Auto and Gen 2/3), tried set the PCI PEG setting to disabled (and every other option), and tried enabling/disabling 4G decoding. In fact, I've tried setting every combination of the above, but to no avail.

I can boot into SimpleMiningOS with 3 cards plugged in to the 1x PCIE slots over powered usb risers, but can not boot with a 4th card. I tried plugging the 4th into both the final PCI-E 1x slot and either of the free PCIE 16x slots, it makes no difference.

All cards are stock. I haven't flashed them with any custom BIOS, and haven't tried over/under clocking them yet.

I know it's not a bad card/riser that's causing the issue, because I tried rotating cards/risers, and each of them work just fine as long as only 3 GPUs are installed when I boot up.

What else can I try to get 4 GPUs booting? Obviously I'd like to get all 6 working, but... baby steps.