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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

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On many systems the memory banks should be installed in pairs (of same size and model). This is due on how chipset access memory (and allow parallel access, so double speed).

According https://www.asus.com/Motherboards/P7H55M_PRO/specifications/ , your motherboard requires pairs of memory banks.

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  • I am reading the specifications and I no specific mention that it requires modules to be install in pairs. I actually see the reverse stated, "Some hyper DIMMs only support one DIMM per channel", which tells installing a single module is supported. – Ramhound Feb 29 '16 at 21:00
  • Added the memory stick info to the main post. I don't think my motherboard reqiures dual sticks because it would work with just 1, but apparently not 3 – BLOO Me-ewan Feb 29 '16 at 21:03
  • @Ramhound right. It seems to support 1,2 and 4 (explicitly) and 3 (on the list of supported memory banks). So now I assume that it is "just" an unsupported memory combination. – Giacomo Catenazzi Feb 29 '16 at 21:10
  • @GiacomoCatenazzi - I 100% agree. – Ramhound Feb 29 '16 at 21:11
  • I agree. It seems that for some reason this motherboard/CPU/chipset/BIOS combination does not accept this combination of memory sticks. You can try moving sticks to different channels or upgrading the BIOS and it might work. There's no fundamental reason it can't work, it just happens not to. And the motherboard documentation nowhere suggests that three memory sticks are supported, though it's a very strange limitation. – David Schwartz Feb 29 '16 at 21:23
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1333 MHz and 1066 MHz are the compatible speeds for your motherboard without over-clocking.

1600 MHz should be over-clocked to use.

Only one option. Install 4 GB 1600 MHz Slot 1, 4 GB 1333 MHz on slot 2 and 3. See if that solves. Mixing two speeds on most systems will automatically default to the lower speed as a whole - Meaning 1333 MHz and 1600 MHz will down-clock to 1333 MHz on the whole.

If that doesn't work, manually down-clock the dram timings of the 1600 MHz to 1333 MHz (comparable to the 1333 MHz 4 GB's timings), check if it boots up successfully, then add the 1333 MHz 4 GB's in slot 2 and 3.

While single 4 GB 1600 MHz will work alone, it might not work together with other speeds due to the over-clocking needed.

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