⚠️ Important Disclaimer
Incorrect BIOS changes can prevent a motherboard from booting. Note your current settings before making changes, and only proceed if you're comfortable resetting the BIOS (via jumper or battery pull) if something goes wrong.
Getting six or more GPUs to boot and stay stable on a single motherboard almost always comes down to BIOS configuration. This video goes setting by setting through the ASRock B450 Steel Legend BIOS, covering PCIe mode, above 4G decoding, and the other toggles that make a multi-GPU mining rig boot reliably instead of hanging on POST.