Update:

I tested each slot with 1 and 2 Sapphire RX580 8GB cards:

   • Slot 1 (closest to the CPU): Doesn't work, eventually boots, but card not usable
   • Slot 2: Doesn't work, eventually boots, but card not usable
   • Slot 1 + 6: OS lock up
   • Slot 2 + 3: OS lock up
   • Slots 1 + 3: OS freezes during boot, but eventually boots, cards unusable
   • Slots 1 + 4: OS freezes during boot, but eventually boots, cards unusable
   • Slots 1 + 5: OS lock up
   • Slots 2 + 5: OS freezes during boot, but eventually boots, cards unusable
   • Slots 3 + 6: Works fine
   • Slots 3 to 6 are fine

It seems only slots 1 and 2 won't work for some reason hmm?

I went into the BIOS and it only shows options for 4 x PCIE slots? (The ports go from 0 to 3 only)

https://www.dropbox.com/s/1caurp0l3hjua62/IMG_9831.jpg?dl=0
https://www.dropbox.com/s/o68c2to1og7su3c/IMG_9832.jpg?dl=0

Are they sharing a bus or something. Do I need to do something special to make slots 1 and 2 work?