I'm posting this in the hope someone else won't suffer the torment I did. Apparently the differences in Xeon processor architecture can cause a change in the memory dimm slot order on the X58 FTW3 board. I have the max memory installed however I started with 8 gigs (two 4g dimms) in slots one and two which detected and ran just fine, however when I added four more to the remaining 4 dimm slots windows only recognized the first 8g not slots 3 4 5 or 6. I even called tech support and got no help there as it was an old board. Interestingly enough whilst searching for a replacement motherboard to use what I had put together I found a post of someone else w the exact same issue. Soultion- he swapped his 4 dimms to slots 3 4 5 and 6. now I had all 6 populated and it only saw the first 8 but I tried anyway w an extra twist. I started w dimm slots 1 and 2, then swapped the dimms from slots 1 and 2 to 3 and 4 and added 2 more dimms in slots 5 and 6, each time booting the machine to let bios and windows see what had been done and see if it had any affect for myself, it did. I then added the final sticks in the 1 and 2 slots and it worked I had all 24g showing up and working in windows. it definitely mad a difference too so it is working. 
 
 BTW to fully test I ran Fallout 4 in ultra settings which I could not do before without death lag in a firefight and it's faster than it was even in a modified high setting before.
 
Currently I'm running a Xeon 5650 w 24g ram and a discrete Nvidia Geforce 1030 and no overclocking. 

Thanks for this info

Might want to sticky this or something. I've noticed a lot of people seem to be grabbing up these old x58s due to the plethora of cheap zeons available and their ability to easily handle the much higher temps involved w overclocking. discounting the lack of a few additional instruction sets an OCd Zeon can give the high end I class processors a run for their money. not to mention their ability to handle more ram.
   I'm pretty sure the mem issue is due to this processor being a refurb, something may have been done to deal w some issue under the chip. I may have to compare this one to it's brother I purchased to test if it was a defunct processor. 
 
Now that I have 2 of them I'm practically jonesing for one of the SR2s or an equivalent...Unfortunately I'm looking for a new place to park myself while I rebuild my trailer into a tinyhome. Materials cost just went through the roof and my current rental costs kill me being able to spend more than I had planned for. I had just got all my solar and power conversion equip together too. Uggh.
 
This machine was going to be one of the centerpieces. I am building the PSU for it to run off DC. No DC-AC through an invertor then back to DC inside the PSU. I took a lesson from a Carver silver series amp I had long ago and built the PCB for the PSU out of a marble slab tile that I sputtercoated w silver I'm going to hand cut the traces and drill pins for wirewrapping components to them. It ought to be great on thermal dissipation. when I get done I'll throw a build pic set in the appropriate forum group. the whole thing will be my table/workbench.

Normally I would consider making this a sticky but due to the age of the processor and motherboard I suspect there are not that many people using it. However I will tag it as an FAQ. 

Works for me. I like to help. I figure at least a few will run into the same issue and not have my perseverance or penchant for finding the needle in the haystack. Even if I find it by sitting on it...LMAO

also would help if you spell the CPU name correctly.
It Xeon, not Zeon. If someone else has similar issue, they might search the forum for Xeon, and they will never find this post.

nekrosoft13
also would help if you spell the CPU name correctly.
It Xeon, not Zeon. If someone else has similar issue, they might search the forum for Xeon, and they will never find this post.

I corrected the spelling

nekrosoft13
also would help if you spell the CPU name correctly.
It Xeon, not Zeon. If someone else has similar issue, they might search the forum for Xeon, and they will never find this post.

Oops must have been due to me writing that B4 the coffee kicked in. I'm shocked I missed that, Ususally I proofread 5 or 6 times B4 I hit post as I never get it quite how I wanted it to look the first time. Interestingly enough I think I remember something in the bios saying the ram was almost at 1600. Not too concerned till I get the liquid cooling system setup for overclocking. I'm going to run the radiator UNDER the tinyhome I'm building this old RV into as it serves 2 purposes, 1) it won't radiate the heat into the cabin in the summer and 2) in the winter it will provide a little heat underneath so the pipes are kept from freezing longer without my aux heater I have underneath needing to turn on. Granted it means I will have to clean it regularly but I built an access door into the skirt.
 
I fixed it BTW

MadmanMac
I'm posting this in the hope someone else won't suffer the torment I did. Apparently the differences in Xeon processor architecture can cause a change in the memory dimm slot order on the X58 FTW3 board. I have the max memory installed however I started with 8 gigs (two 4g dimms) in slots one and two which detected and ran just fine, however when I added four more to the remaining 4 dimm slots windows only recognized the first 8g not slots 3 4 5 or 6. I even called tech support and got no help there as it was an old board. Interestingly enough whilst searching for a replacement motherboard to use what I had put together I found a post of someone else w the exact same issue. Soultion- he swapped his 4 dimms to slots 3 4 5 and 6. now I had all 6 populated and it only saw the first 8 but I tried anyway w an extra twist. I started w dimm slots 1 and 2, then swapped the dimms from slots 1 and 2 to 3 and 4 and added 2 more dimms in slots 5 and 6, each time booting the machine to let bios and windows see what had been done and see if it had any affect for myself, it did. I then added the final sticks in the 1 and 2 slots and it worked I had all 24g showing up and working in windows. it definitely mad a difference too so it is working. 
 
 BTW to fully test I ran Fallout 4 in ultra settings which I could not do before without death lag in a firefight and it's faster than it was even in a modified high setting before.
 
Currently I'm running a Xeon 5650 w 24g ram and a discrete Nvidia Geforce 1030 and no overclocking. 

Ohhh, long time, I don't enter this forum. I know that is a bit late but this happened to me around 2010 when I purchased my X58 SLI Micro, but in my case, the problem was that the pressure of the heatsink was bedding the board a little causing the memory to detect less and the solution was to release the pressure over the processor socket.
But there is one thing I didn't understand in your post. You said that you have the Max Memory Installed. Did you refer to the number of memory modules right? I still use my X58 SLI Micro and I have 48Gb Ram working just fine for more than six years now.

That comment was in regards to no updates to the user manual/specs stated by EVGA.  I had no idea you could put 8gb dimms in the machine and it recognize them. The manual says max 4gb dimms so that was the info I was running off of, or in other words GIGO. Garbage in Garbage out. Machine still running great BTW.