I thought the boards would be 1 2 3 left to right or right to left.
Not the case. Seems they can be in any order.
For me it is trial and error to find sick or dead boards.
Dead boards are easier, little red light is out.
When they are sick the red light is still on. So I pull hash cables one at a time and restart until the sick board goes away.
I have spent quite some time troubleshooting only a few dead boards myself. While I have had only a few boards go dead I will say that the few I have tried to fix had no rhyme or reason to the fixes. The order of boards to I/O is weird.