Setup:

i7 920, c0 stepping, stock clock for troubleshooting purposes
G-skill 9gb (3x1gb, 3x2gb) ram 9-9-9-24 1.5v rated for 1600, currently clocked at 1066 for troubleshooting purposes
EVGA X58 SLI

Additional specs:
Corsair HX1000
260GTX
Thermalright Venomous-X
3x 74gb Raptor in raid-5
1x 1.5tb storage drive
2x dvd-r drives

Originally, I discovered the problem when I received this board via RMA.  I was booted into windows when I realized I had only 6gb of ram showing available.  A quick check into resource manager showed 9Ggb installed, 3gb hardware reserved, and 5.99gb available for use.  CPU-Z confirms that there is indeed 6 dimms installed, and properly reads each slot.

This led me to test each memory slot individually, and leads to this result.

If i have 1 dimm in slots 1,2,5 or 6, the system will boot normally.  However, if I have one dimm in slots 3 or 4, the system will not post.  If I have ram in slots 3 AND 4, it still does not post.  If I have ram in ANY combination that includes 1 and/or 2 and/or 5 and/or 6, the system will boot normally, but the ram in slots 3 and/or 4 will show as "hardware reserved" and is not detected in the bios.  CPU-Z and windows still recognise it as being installed.

e.g.:

I have 2gb in slots 1,3,5 - Windows sees 6gb installed, 3.99 available
I have 2gb in slots 1,5 - Windows sees 4gb installed, 3.99 available
I have 1gb in slots 1,3 - Windows sees 2gb installed, 0.99 available
I have 1gb in slots 1,3,5, 2gb in slots 2,6 - Windows sees 7gb installed, 5.99 available

I've tried multiple configurations to confirm.

Additionally, I've tried all the dimms in "working" slots, and verified via memtest that they are all functional.


Additional 'possible fixes' I've found on the forum include reseating the CPU, cold start vs hot start, memory low max, and  msconfig>boot>advanced>maximum memory.  None of these fixes have fixed the issue for me.

I've also tried two bios releases (current, and 2nd oldest) and my issue persists.

Changing voltages for ram, QPI, cpu, etc. does not fix the issue.

Changing ram times to be even more relaxed does not fix the issue.

Changing the "pressure setting" on the venomous-x does not change anything.

Last, but not least, I have verified via a borrowed Gigabyte X58a-ud3r that my CPU and RAM is fully functional; everything works perfect on that board, including all 9gb being detected properly.


Please help!     

I've read through quite a few, including one that was 5 pages long, hoping to get some insight.

My original board never had this problem :( 

OP how many passes of memtest did you let it make?  Also what bios version are you using?

I had the same problem with a new E760. 6GB installed, BIOS showed 4GB and Win7 showed 6GB installed with 3.99GB usable.
 
Try testing each slot with one stick at a time. I've seen a rule of thimb in another thread in this forum that advised if BIOS shows less then the amount of RAM installed it's likely to be the board. This was true in my case and the boatrd has been RMA'd.