SOLVED! Finally 
First I had to clone OS from M.2 disk to SATA SSD disk to free up PCIe lane. That was the catch.
Then I pulled out all GPUs, ONE BY ONE to determine, whether it is the catch on PCIe slot or on PCIe-to-USB adapter. And it was PCIe! Simply could not handle more than 3 GPUs directly attached to PCIe slots.
So I used:
- 3 GPUs via risers directly attached to mobo
- 1 GPU via M.2 PCIe adapter
- 4 GPUs via PCIe-to-USB adapter
Plugged in then 1-by-1, rebooted and WAITED few minutes after each reboot, until Win10 settled down. If GPU was not found, rebooted again. After 2 hours rebooting and plugging-in finally all 8 GPUs running
First I had to clone OS from M.2 disk to SATA SSD disk to free up PCIe lane. That was the catch.
Then I pulled out all GPUs, ONE BY ONE to determine, whether it is the catch on PCIe slot or on PCIe-to-USB adapter. And it was PCIe! Simply could not handle more than 3 GPUs directly attached to PCIe slots.
So I used:
- 3 GPUs via risers directly attached to mobo
- 1 GPU via M.2 PCIe adapter
- 4 GPUs via PCIe-to-USB adapter
Plugged in then 1-by-1, rebooted and WAITED few minutes after each reboot, until Win10 settled down. If GPU was not found, rebooted again. After 2 hours rebooting and plugging-in finally all 8 GPUs running
Good, I'm happy for you. Thanks for actually posting the results after you resolved it. More people oughta do this, it's useful to anyone else who stumbles on the thread.
