Hi,
so I found a possible solution for the said problem.
ProblemGigabyte Fintech 12 GPU B250 mining mainboard does not boot / post after connecting more than 9 GPUs.
It does not depend on which PCIe slots you use, just disconnecting any card helps, as long as there are no more than 9 connected.
Intro.. As stated, if the problem solves by disconnecting any of the cards so that there are only 9 connected, you can quickly check if you have that specific problem. There is still a possibility, that your PSU is the problem, especially, if you built a new rig and boot up for the first time. Maybe your rig just draws too much power.
.. It still might be a riser, although you ruled that one nearly out by confirming it is not related to specific GPUs.
.. Might still be a defective component on the board.
Steps done before in no particular order, that were not related to the solution and did not solve the problem.. Switched RAM and CPU
.. Switched RAM slots
.. Loaded defaults in BIOS
.. Reflashed recent BIOS
.. Flashed old v3 BIOS
.. Checked every setting and deactivated all unneccessary devices
.. Mining mode is enabled, 4G decodeing is enabled, primary video device is integrated GPU etc...
SolutionThere seems to be a problem with some boards and also with the new 4a BIOS version. In one case it helped to just flash back version 3, but in another case, that did not help.
The solution was to flash all BIOS versions from the beginning in the right order. You need to download all the files from Gigabyte and just put the BIOS files on a USB stick in order to use the on board flash tool EZ Flash from within the BIOS.
Important: Make sure to always chose the "full" upgrade of the BIOS, if given a choice by EZ flash!
So I started with 9 cards connected because I was lazy and did not want to disconnect every card. Flashed F1, then rebooted into OS, shut down. Flash F2b. F2b for some reason did not boot at all with GPUs attached. So I needed to disconnect all cards to have it boot into OS. Shut down. Flash F3. At this point your board should work fine with 12 cards again with the right settings. Do not flash F4a! This will bring back the problem. For science I did. If you do not mess up with a lot of things, flashing back to F3 fixes this again at this point.
ConclusionSeems that there goes something wrong in the BIOS that gets solved only, if you flash all versions in correct order until V3 (always chose complete flash!). F4a seems to be one cause of the problem. But also boards that do not have F4a installed seem to have this problem, also fixed with the provided solution. At least for boards I had so far.
I hope this helps and I would appreciate feedback! Btw. 300 posts