Issue Running 4 Cards

[hermitagetool]

I'm having one heck of a fun time trying to get more than 3 cards running on my various mining rigs. I'm using 3 Biostar A870 boards with both powered and unpowered PCI-E x1 and x16 extender cables.

2 boards are populated with 4 reference 6970s each, and are powered by Kingwin 1000 watt 80 plus gold PSUs. Running up to 3 cards works w/o issue, but using 4 sends my hashrate through the floor. MSI Afterburner also stops working.

The third rig uses 4 1 GB 6950s, and an NZXT Hale 850w PSU. This rig won't even detect 4 cards, but like the first, using 3 is fine.

What's happening, and what can I do to fix it?

[cicada]

Assuming it's not the same '4th card' that isn't working among all three machines, it's probably a BIOS config issue.  You may have to reduce port speeds for the 16x slots when all four slots are occupied, for example.

I did a quick search and didn't find anything relating to the motherboard not being able to utilize all 4 PCIe slots, but it's common for board to disable a 1x slot when both 16x slots are occupied, to provide greater bandwidth to the second 16x.

[cirz8]

Just for fun, in the BIOS, disable sata/sound/everything else not needed, and boot up using a linux miner from either IDE or USB.

[Sideways]

The bios pages are sometimes the most useful. On most boards you can disable 'quick boot' or spashscreens and see what bios detects. My one msi board will even tell you which slot and what bus speed it gives each device. So in there I can see if it sees all 4 cards, and what slot they are in. If bios only sees 3 cards, no OS will.