My system has 6 DIMM slots and is Triple Channel. . The 'center bank's, DIMM 3 and DIMM4, are not recognized in the BIOS. But they are 'seen' by the system, however the memory located in those banks are flagged as "Hardware Reserved". I'm assuming this is done to prevent the OS or applications from trying to write to those address blocks, erroring out (because the banks are faulty), and causing a BSoD or data lose prior to being written to the harddrive.
I've tested all three RAM Sticks in DIMM1 (aka DIMM0 in some software) and all the stick check out both in the BIOS (sees all 4GB of each stick) and in MemTest86.
Same goes for all six of the RAM I tried (2 sets; (1) 6GB, (1) 12GB). DIMM1,2,5,6 all detect the sticks in BIOS and MemTest86. Anything put into DIMM3,4 are not seen in the BIOS or in MemTest86.