I have a similar issue with Gigabyte X58A-UD3R (rev.2.0 FB BIOS) + Corsair XMS3 7-8-7-20 1600Mhz DDR3 Triple Channel Rams.
I set up my system yesterday night, it recognize only 4GB on bios but..
Identifies 6GB triple channel installed on GPUZ and 6GB installed (4GB in use) in Win 7 system.
DIMM 3_1 , 3_2, 3_5 --> shows up 6GB dual channel all fine
1, 5, 6 --> 6GB dual channel all fine,
2, 5, 6 --> slot 2 does show the RAM but then empty slot 4 enables for it --> 4GB
1, 3, 5 --> slot 3 does show but slot 4 enables --> 4GB
3, 5, 6 --> slot 3 does show but slot 4 enables --> 4GB
Several combinations of the modules does not change anything, so i guess its not an individual module problem
Tried couple of BIOS'es , Checked for bend CPU pin but cannot see anything.
I dont think i did harm the mobo , installation was pretty neat.
tried several combinations of overclocking and overvoltaging of the rams but did not change anything.
im pretty frustrated by this issue of Gigabyte mobo. there are many who has other versions of GB X58 boards and almost all has the similar problems.
any help is welcome.