gabryrox -
I have many H81 BTC mobo with 6xGPU and 7xGPU configs with no problems.
In my case, I got my multi GPU tutorial from BBT - Bits Be Trippin.
You need to ensure that this particular mobo (v1 and V2) has the molex powered directly from PSU.
In addition, all GPUs need to have powered risers.
The errors you are getting, IMHO are power related issues... there just not enough power for 6 GPUs.
Also your PSU need to be large enough to cater for all 6 x GPUs plus you mobo, etc. with 20% to spare (good rule of thumb to ensure safe zone and also longevity of your PSU).
I had a few suppliers from China via AliExpress on the powered risers - ensure your get the version 3 with USB3 type - I have very little issues with those. Avoid the ribbon type risers because the solder works on those are back-alley quality and are a fire-hazard.
Best build for multi GPU is Windows 7 in my opinion, you may need to use the 6xGPU registry mod hack utility to get rid of the Error Code 43, but no rocket science, it's quite straightforward. If you want lazy shortcut and want Windows then just go for Win10 to get all 6xGPUs visible in Device Manager.
For mining (not gaming), 16x PCI is irrelevant, the USB risers are 1x (...4x ??) and enough to carry DATA between OS and the GPU. Unlike high end games, it will utilize the PCI bandwidth on all cylinders to use all the cool features of the GPU. In mining, a big chunk of the GPU's greatness is likely not used because mining need at best the core GPU cycles and the high speed memory. Thats why miners are modding the ROM and taking out the unnecessary stuff -- this ritual is popular in hard core mining.
Afternote:
While I have the multi-GPU config still in my mini-farm, all my new builds are riser-less on based on Biostar Z170 4xPCI slotters. Keeping it simple as possible.
Thanks Citronik for the thoughtful and helpful information... i really appreciate it! I will respond to all your points as some i have done and some yet to do:
1- I do have the additional molex plug powering the mobo. I knew I would need to do this going in so after getting that message I think after installing GPU #2, I plugged that additional molex into the mobo.
2- I am using all powered USB risers at this point, in x1 slots 1, 2, 3 & 5. I got most of them from the same Amazon seller and didn't have any problems with 3 of them on rig #1 and the first 4 on this rig. However, I do have 1-2 extra ones lying around, along with 3-4 additional USB-3 cables, so I imagine it's a good idea to try all of those before proceeding with further trouble-shooting.
3- My PSU is a Rosewill 1200W Quark Series Platinum. With 4 GPUs running on Claymore ZEC miner V8, I am hitting total system wattage peaks in the 600-610W range. This includes the CPU/Fans/etc, which use about 60-70 watts, so even adding 2 more 470's would put me at just under 900W max... so, less than 75% of its total capacity. Based on this, I don't believe it is a total power issue causing my problems (unless I am really missing something).
4- I'm already on W7 and would prefer to avoid W10 if possible, so I am considering that registry hack you mentioned. I actually read about it over on the ZEC mining forum... at least I assume it is the one you are talking about. Would you mind checking to see if this is the same one?
https://lbr.id.lv/6xgpu_mod/Since this involved messing with the registry I don't want to take any chances, but if this is the same version you are talking about and you say it's relatively straight forward to implement, I will give it a shot.
Hoping/assuming this mod will solve the error 43 issue in slot #6, I still have that pesky issue with the lone x16 slot on this board. Have you ever experienced anything like that? (not being able to get past post with anything plugged into it via USB riser, including setting PCIe to Gen1 (from auto) in the BIOS). Note that I tried to use this slot several times before getting my rig up to 4 GPUs, so I don't believe it's the number of GPU limit issue causing this problem.
I also read that some people have had success by turning off the on-board video in the BIOS. Not sure that was meant to fix this exact issue, but maybe it's worth a try? Not sure exactly how to do that... will probably take me at least an hour to find as does most anything I try to find in the BIOS lol.
Thanks again for the tips... hoping to get this rig up to 5 GPUs soon, ideally in the x16 slot, so I feel comfortable enough that it will take 6 GPUs to buy the 6th.