I have a 2010 Mac Pro 5,1 with a quad 2.8 GHz CPU. The backplane board is dead. I want to salvage this machine, cheaply. But 5,1 logic boards are expensive ($300). A 4,1 board costs only about $120. It's the same part number, but EEE code is BH9, different from 4,1 boards. Is there any chance that a 4,1 (2009) board will work in an otherwise 5,1 machine? This is a 2010 with the lowliest old-style CPU, so my thinking is that it MIGHT not need the new CPU support of the 5,1 backplane. Would the machine work with a 4,1 backplane with upgraded firmware? Would it boot enough to upgrade the firmware (given the old-style CPU), or would the firmware upgrade have to be done beforehand on a 4,1 machine? Finally, what is the underlying difference between between an upgraded 4,1 backplane and a genuine 5,1 backplane? I heard something about the fans running full all the time, because of different SMC firmware. Can this be upgraded too? I've read that a 4,1 backplane is forever a 4,1 even if upgraded? Is this true, and why? Thank you for any tips, or any link that explains these issues in detail.