I upgraded my Mac Pro 5,1 dual CPU board (4 core 2.66), to another dual CPU board (6 core 3.46). With my old board, all 8 of my 1333 8GB DIMMS were recognized (64GB). But after upgrading the new board, only 6 of the 8 DIMMS show up (48GB). No matter how I set the DIMMS up, only 6 are recognized. All ram slots work, because I tested them by pulling 2 DIMMS out. And used the 2 remaining ones on one side, in all 4 slots. And each one was recognized. Anyone know why this is happening? Thanks. Please note, the newer dual board with the 6-core, wasn't originally 6-core. It was also a 4-core. But I believe it was a 2.8 instead of 2.66. The chips were just upgraded, and I swapped out the boards. Other than the Ram issue, everything has been running well.