See title. I have 2x 32gb dimms installed with my 12900k. Been running great for a couple years. I also have 2 nvme drives (top 2 slots), my 3090Ti (top x16 slot), and a USB expander in the bottom most x4 PCIe slot.

I want to add 10gb networking, so I added an Intel 10gb card I pulled working from another server to the previously empty x8 PCIe slot. When I boot, one of my ram sticks is no longer detected and it shows only 32 GB available. The system otherwise boots just fine. If I remove this card and boot again all 64 GB are available. If I reinstall the 10 GB card and remove the USB expander, I once again see only 32GB of RAM.

Is there some resource limit that is happening that would cause one DIMM to be unaddressable when installing this NIC?

which Z690 MB do you have?
OS?
PSU? - maybe not enough power?


Is the AUX power socket on your MB, connected to the PSU to give more power to PCIe?
 

• Z690 Chipset Motherboards
  • Support Manual for EVGA Z690 DARK K|NGP|N (121-AL-E699)  (page 14, Item #9, located lower left of MB = PCIe Aux Power Port)
  • Support Manual for EVGA Z690 CLASSIFIED (121-AL-E698)  (page 12, Item #21, located lower left of MB = PCIe Aux Power Port)

I’ve got the Kingpin mobo.

Psu is a 1000W Seasonic titanium, so power should not be an issue.

No, the aux socket is not connected. In fact, I’d never even noticed it was there. Let me see if I can get a 6 pin connector over there and report back.

mokrunka
I’ve got the Kingpin mobo.

Psu is a 1000W Seasonic titanium, so power should not be an issue.

No, the aux socket is not connected. In fact, I’d never even noticed it was there. Let me see if I can get a 6 pin connector over there and report back.

Alright, ran another 6pin PCIe cable back there and tried again. Same results this time. No matter whether the USB card is in the slot below or not, the presence of the 10gb NIC causes one of the memory sticks to not be read.

Hmm. Now that is an odd one. You should contact evga to see what they have to say about this. Maybe some odd bios bug?

Fast BOOT is Off in BIOS?

Not sure if fast boot is on or not - I'd have to check. What does that have to do with this (genuinely curious)?
 
I also did some more reading about my NIC. It seems it's a PCIe 2.0 card, not 3.0. I am buying a PCIe 3.0 x4 card (which I needed to do anyway because the fan in this one turns out to rattle), the Intel X550-T2. When that arrives, I'll try again.

mokrunka
Not sure if fast boot is on or not - I'd have to check. What does that have to do with this (genuinely curious)?
 
I also did some more reading about my NIC. It seems it's a PCIe 2.0 card, not 3.0. I am buying a PCIe 3.0 x4 card (which I needed to do anyway because the fan in this one turns out to rattle), the Intel X550-T2. When that arrives, I'll try again.

Anytime you add or remove hardware; "you Want the MB to take inventory" - then the proper resources are applied - After the hardware change is recognized. 
Your PC always has power (short of unplugging it) & that "soft off state of a Windows shutdown"  ---> prevents proper resource allocations needed by hardware changes.
 
Windows Fast Startup causes its own issues when you change Drivers or Hardware -- because Win Fast Startup just uses the File from the Previous good start to launch the next session of Windows ... & this lets Win disregard any & all changes to the PC. 
 
Easiest fix for Win Fast Startup - Hold the shift key down before you select shutdown & keep the shift key down until the PC is totally powered Off.  That simple step, erases the last previous good start file, this forces Windows to look at everything again & properly allocate resources & recognize new drivers
 
Both of those steps are considered a Best Practice
 
This may or may not help with the current strange issue you are seeing: hardware added & RAM disappearing issue
 
They are a simple set of steps, costs nothing, might help & cant hurt to do, kind of a back to basics thing
 
 

Ok, I tried turning off fast boot, and holding shift while shutting down and doing a cold shutdown/reboot.
 
I tried this with my new, Intel X550-T2 PciE 3.0 x4 card in both available PCIe slots.
 
I get the same result, one DIMM goes undetected and my RAM goes from 64GB to 32GB.
 
I'm stumped.