So, I have a Core i7 750 installed on an ASUS P7H55-M PRO using Windows 10 x64 Pro.
I had 12GB showing all ok. I had to replace the 2x2GB for a 1x4GB. Problem is now I only see 8GB instead of the 12GB it had before, everywhere in windows (task manager, computer properties, systeminfo.exe, dxdiag.exe etc).
In CPUz I see them as a single channel 12GB, RAMMon displays all the sticks properly.
P.S, I don't see the "usable" part, just the 8GB of ram.
EDIT: The memory sticks are 2x4GB G.Skill 1333Mhz f3-10666cl9-4GBNT 1x4GB corsair "vengeance" 1600Mhz CMZ4GX3M1A1600C9. My BIOS also shows 8GB usable RAM.
The old ones were 2x2GB Kingston 1333Mhz model kvr1333 something