Having dealt with the b250 mining snob for several years ( I own two but only use one at a time. ) I’ve learned that if you’ve tried all the things you’ve mentioned above; (and trying running hiveos on a different media/reflash) and its not your PCI-X risers, sounds like its broken or you’ve overloaded your electrical circuits. Maybe take a picture of it? Did you plug in the molex connectors into the motherboard? (DONT) I read somewhere this can brick your motherboard if you do so in conditions where its not recommended (I believe if you are using less than three graphics cards, you ‘need’ the molex connectors.) I’ve never run that few so I cannot confirm that but I remember reading about it in the manual and being confused.
I myself have had this problem many many times and I never really truly figured out what triggered it, but it somehow resolved itself after letting the thing ‘rest’ for a few days. But I’ve learned when this happens its a result of a combination of things stemming from me being lazy about my cable management, improper power balancing, mismatching power supply connections (using PCIX power from psu A and riser power from PSU B) and properly managing airflow.
I’ve set my mb on fire with a improperly plugged in pcix slot, melted the slot, but the motherboard still works fine (just not the burnt/melt port or hey maybe it would but I dont want to chance another fire) So maybe theres something up with your electricity cause these boards are tanks.