Just did a 4,1 to 5,1 upgrade on my 2009 Mac Pro running 10.11.6 El Capitan. All is well EXCEPT that RAM slot 1 isn't recognized. Before: 2009 Mac Pro 4,1 2 x 2.26GHz quad-core After: 2009 Mac Pro 5,1 2 x 3.33GHx (x5680) hex-core Firmware update went fine. Both CPUs are recognized in System Report. RAM slot 1 is not recognized, and red LED is lit on the logic board, between the heat sinks, next to CPU B. Tried: resetting NVRAM, resetting SMC, re-seating RAM, swapping RAM with slot 2 (the RAM is fine). Last week when I repaired my Northbridge heat sink, a diagnostic LED on the logic board lit up, and I got it to go away by tightening the heat sink bolts. I didn't know what the LED was indicating, but it was in the same general location between the heat sinks. (I think the CPU LEDs are on the sides.) Tonight I tried tightening the heat sink bolts a little, but I'm hesitant to go further. Apple Service Guide says "finger tight plus 1/4 turn". I think I'm there, or even a little past. Suggestions? Thanks!