I recently helped a few friends install an ETH mining rig that they'd built on a wobbly Taipei rooftop in the middle of earthquake season. With a motherboard specially designed for mining, SMOS Linux, 3 GPUs and a fourth on the way, they seemed well set for success.
Except, only one GPU would be recognised by the mining program.
All GPUs lit up and spun up their fans. Each of them would also happily connect to our monitor. ls /dev/nvidia* would show three GPUs present. We were dumbstruck because the GPU that would work was one of two identical models in the rig: MSI GTX 1060s. We couldn't figure out why one of them was any different, but Nvidia's System Management Interface (nvidia-smi) indeed returned ERR! instead of the name of two of the cards.
When we left only one of the non-working cards in the system, we'd be told No AMD OPENCL or NVIDIA CUDA GPUs found, exit. Then we found out that one MSI was manufactured in 2017 and the other in early 2018. So something must be minutely different