I'm not sure how to help you. But have you checked out your Service Manual for your Service Tag Number,  The section on upgrading RAM.  There seems to be some incompatibility between your current hardware and Windows 10 Pro workstation. I can give you a link to an excellent Windows ten forum. I think if this is a windows 10 issue several people in that forum will recognize the issue and show you the workaround.Windows 10 Help Forums (tenforums.com)

Sorry, I can't be more helpful as I have no experience with 10 Pro for workstations. I am very interested since I just ordered a workstation with the same OS and I will be doing some upgrades right away RAM and SSD's, 

I'm really thinking that this is a security issue with you changing Hardware but I don't have the knowledge to understand it myself. there are some very sharp people in the forum I linked. Good luck, I'm following this thread.

Thank you I'll give it a try. 

Update: dropping it to 64GB (2x32GB sticks) allows windows to install and boot normally (with improved performance over 4x8GB). Sticks are spec'd at 2666MHz but according to BIOS are only running at 2400Mhz.

Will welcome any advice.

Sounds like the voltage needs to be increased to get them up to 2666. Look at the chart for these sticks as to what voltage they need to run at 2666 Hwinfo64 might help you read what is actually happening?

I can't edit my original Reply, so while voltage does affect your RAM Speed so does Timmings see Hwinfo64 read out on the RAM on this old Z97 Motherboard. So to get the full speed you need to check the voltage requirements for your motherboard and required trimmings to get full speed performance.

So whatever your original RAM Speed was if they were working at full speed then match that for your upgraded RAM. it all rather complicated and takes some research for proper RAM purchase.

Thank you so much for your replies!

I tried putting the other 2 RAM sticks in after reinstall to bump it back up to 128GB, just to see what would happen. It failed again and hung on the spinning black dots. I then removed the RAM to see what would happen, and it booted to Windows again without reinstallation at 64GB.Windows does not like 128GB for some reason. 

The RAM I bought is actual Dell RAM, p/n A9781929 (https://www.dell.com/en-us/work/shop/dell-memory-upgrade-32gb-2rx4-ddr4-rdimm-2666mhz/apd/a9781929/memory, look towards the bottom for 5820 as a supported system).  I installed them according to the user manual, and the sticks showed up in the BIOS as taking slots 1-4 (i.e., all the slots with white clips).

Clearly there is something that needs to be updated on the motherboard, and I really hope someone from Dell responds to this thread. This machine is only from 2018!

Yep, windows 10 pro workstation is supposed to take up to 6 TB of Ram, so it must be a Dell MB Issue. Have you tried Business tech support it's somewhat better than what ordinary people have to put up with. I had to contact them a couple of times both times it took me several times and different techs until I got one that actually had any idea about the product. However, that was with my 13" Inspiron 3786 the business tech really had not much experience with that nonbusiness notebook.

Thank you all so much for responding!

1. 32 GB DIMMs are only supported on systems using Xeon W Series CPUs.

I have an Intel Xeon W2104 CPU@3.2Ghz, so check.

2. Check the P/N compatibility. bank/rank configuration may be important, especially with big modules.

According to this link here (https://www.dell.com/en-us/work/shop/dell-memory-upgrade-32gb-2rx4-ddr4-rdimm-2666mhz/apd/a9781929/memory), Dell P/N A9781929 is compatible with 5820T (scroll to the bottom). I have followed the instructions in the User's Manual and at this link here, which clearly shows to install a 32GB RAM stick in Slots 1,2,3, and 4 for 128GB. See my BIOS picture below indicating that the BIOS reads them properly (note though they are only at 2400MHz, not 2666MHz):

3. After failure, remove the upgraded modules and check the bios logs for RAM errors. Sometimes, just one module can cause problems, and that will be logged on RDIMM/Xeon systems.

I looked at the BIOS logs and only saw entries that system memory had changed.  That is within the BIOS, correct?

4. Sometimes (in rare cases) new ram sticks need contacts cleaned with 99% alcohol. And very often the RAM slots need careful cleaning with hard(not waxed) paper and vacuuming.

Haven't tried this yet specifically, but I did use compressed air to clean the inside out when I tried installing the 4 RAM sticks all at once. May try vacuuming the slot.

Here's my opinion, someone talk me out of it: I was able to install Ubuntu 20.10 x64 to this very same computer, and all 128GB RAM worked:

Therefore, I don't think it's the RAM. I think it's some byzantine Dell setting that is not allowing Microsoft Windows 10 Pro for Workstations to boot properly when the RAM is installed. Currently, I have:

- Secure Boot turned off

- Legacy ROM's turned off

- UEFI enabled

Hoping @DELL-Alasdair R, @Dell-Alan D, @DELL-Justin C, or @Dell-DylanJ to comment here please. These are supported Dell parts and Dell configurations on a system that's only two years old.

Still trying

Just to test, I have now tried 64GB configurations by removing one set of 2x32GB sticks with the other 2x32GB sticks.  Both boot into Windows no problem.

Is this a memory channel issue?  Should I be doing something other than slots 1-2-3-4?  Like, say, 1-2-5-7?

Actually, RAM is set up in Slots  1-3, 2-4, etc,That way they work in Dual Channel 

INSTALL windows without the extra ram.

Then put ram back 2 sticks at a time.

Windows has had installation issues with large amounts of ram for many decades now.

Take out all the ram and ONLY put back the 2 offending sticks

Does it work then?

Your issue sounds more like trying to mix ranks and vendor and density and speed.

This is not supported and rarely works.