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The reason might be due to rank limitation. DDR3 R-DIMM CPU can control max. 3 channels. Each channel controls 3 DIMMs, and supports up to 8 ranks. If we want to install Quad rank modules to each channel, then we can only install 2 (4 ranks*2 = 8 ranks). If we want to install modules to a full load, we can install dual rank modules to each channel  (2 ranks*3 = 6 ranks, which is within 8 ranks limitation). Installing 3 Quad rank modules to each channel will result in malfunction (4 ranks*3 = 12 ranks, which exceeds 8 ranks).