Alright, so I did a mild overclock on my i7 930 of 3.2ghz. Just noticed today that it only showed 3.99 out of 6gigs usable. So, I went and tried increasing the voltage to the memory and cpu vtt. The bios then recognized all 6 gigs. Went back into windows, 2 gigs were msising again. Went back into the bios, and it only showed 4 gigs.
So, I reset to defaults and windows sees all 6 gigs. Tried again, same problem. Then, I reset to default, same problem. Went back into bios and reset to default a few times, and windows sees all 6. If this makes sense. Basically, the bios showed 6 gigs after increasing voltage, but windows didn't see it. Went back to stock, windows didnt see it. Now windows sees it again.
Hi newcomer and welcome to the Tom's hardware forum.
What mobo do u have? Do this:
Run msconfig, boot tab, advance options and check the part that says "Max memory" shouldn't be selected.
Did all that. It only happens when I overclock. Looking at some forums, it's apparently a widespread problem. At default settings, everything is fine. When I overclock, even a very small overclock, 2 gigs of memory disappear.