Good troubleshooting by the way.

Fast startup only starts after POST if I have that right.

Due to your extensive troubleshooting, I don't have any new ideas, except one.  Try the "Get help now" button on the lower-right of the screen.

I did perform a RTC reset a day ago as it was advised in other cases.

https://www.dell.com/support/kbdoc/en-us/000126023/how-to-reset-real-time-clock-rtc-to-recover-your-dell-desktop-system

But unfortunately, the problems remains.

I hope the Dell engineers can make a BIOS update in which this issue is resolved.

I have gotten in contact with Dell support via Get help now, but it took hours to get a reply and when I got in contact with a support agent I only had a short amount of time before I had to go myself. I linked to this forum post.

I have been in contact with Dell support. They instructed me to run ePSA diagnostics which shows an error code 2000-0145 on a Corsair MP600 Pro NH 8TB SSD that I added myself to one of the m.2 2280 slots. Under Windows, with SMART diagnostics and the Corsair SSD toolbox the SSD shows no problems and works fine.

But as this is a part that was not installed by Dell it is not supported so I will need to test if the uncommanded boot problem persists if I remove this SSD.

The uncommanded boot problem ONLY occurs when my user-added 8TB Corsair SSD is present in the system. So I think there is a compatibility problem between the Dell Precision 7875 and this Corsair MP600 PRO NH SSD. When The SSD is removed, my workstation stays off as it should when shut down.

Glad you found the problem.  :)

Crucial is a good brand if you want to try it.  I've used their SATA SSD's myself.

@bradthetechnut​ Thanks. Dell also 'helpfully' directed me to their sales agents to suggest me buying a compatible SSD for my system but I thanked them for it. I'm not interested in buying another SSD at this time. Dell charged the same price I paid for my 8TB SSD for a 4TB one and did not offer an 8TB m.2 SSD in their configurator. Crucial does not seem to have a 8TB m.2 2280 model for sale. So I will stick with my 8TB Corsair. It is lightning fast and completes all SMART / surface tests without problems. Perhaps the compatibility issue will be resolved later of more customers ran into similar problems. The entire issue has given me some mixed feelings. I fully understand that Dell support is limited on third-party hardware issues. But at the same time: A PC like this is an open platform. I should be able to add third party SSD's without these issues.

I made a ticket with Corsair support for this issue. Here is their answer:

<Private data removed from public view. DELL-Admin>. To add additional comments, reply to this email.

<Name>. (CORSAIR Support) (Corsair)

Apr 1, 2024, 11:30 PDT
Hey <My name>,
 
Thanks for reaching out, sorry to hear about the issues you are experiencing with our storage drive. Unfortunately the reality here is that the update will need to come from Dell. When you mentioned "Dell BIOS update is needed", we believe this is correct. There is nothing unique with our drives in comparison to our competitors in the sense that you do not need "iCUE" for example to run the drive normally.  If the motherboard has Gen 4 M.2 slot it should work normally. We have had issues with other customers for example who purchased our DDR4/5 memory to be used with their Dell Workstation and they mentioned it working normally until there was a BIOS update that removed compatibility with third-party products. It seems like Dell's refusal to help you in this case may be a result of a similar scenario here. If the drive works normally and there shows no SMART errors then the issue likely will be a Dell Issue. 
 
Best Regards,
<Name>
Technical Support Specialist