Elitedesk 800 G2 GPU add-on / weird POST issue

[JakeSnow]

Had the device up and running okay with a graphics card in the x16 slot. No problems with the device other than some POST code errors from putting the device into a different case to accomadate some extra hardware (USB #2). I decided to update the BIOS to the latest available and see if HP added any 'halt on error' options. I don't remember what BIOS I was on prior to updating and the backup of the BIOS was lost (more on that in a minute).

After installing the BIOS the computer restarted and went to a black screen. I gave it a minute, shut down the PC, and didn't think about it much more. I was about to tear down to change some things, and, admittedly, I was in a hurry.

Here's where it gets weird.

After changing some things around I needed to boot the system into Windows for a test. Without any major changes other than removing the GPU, Windows was failing to boot. The device was giving a "no boot device" error. However, it was able to see UEFI diags on the same drive. After way more troubleshooting than I should have done, I chalked it up to me being in a hurry, backed up *most* of my data (I forgot the BIOS backup), and reinstalled Windows. 

Alright, so now we've got a computer that boots to Windows using onboard graphics and performs as intended. 

So I set everything back up as it was before the teardown. Device no longer loads. No POST. No display. Fans and single beep. 

Removed GPU. Boots, POSTs, Windows.

Installed known good GPU. No POST. No display. Fans and single beep.

Removed GPU again. Boots, POSTs, Windows.

Checked BIOS settings. The only options around the graphics are turning off the slot entirely (checkbox just like for USB, other PCI slots) or changing the amount of onboard GPU memory available. 

Reverted device to completely OEM / stock. Everything working. Won't POST with GPU. 

Flashed BIOS again. Same result. Flashed BIOS back to Jan '19 '18. Device now POSTs with GPU, but does not display until Windows is logging in. Still no manual settings for integrated graphics vs PCIe in BIOS.

Flashed back to (I think) Nov '18 '17. Same thing. 

The device POSTs and runs Windows fine, save for the fact that I don't see any of it happen. I can't load to BIOS as I won't see video through the GPU or the onboard graphics outside of Windows, unless the GPU is pulled. So if the switching option in BIOS is only available when the GPU is plugged in, I think I'm SOL. 

The Intel onboard graphics are functioning in Windows. They show up in device manager and if I plug in a second monitor both the GPU and the onboard work. It's my understanding that this wasn't possible on this machine (but maybe that's part of the BIOS updates and I'm just reading outdated posts).

I've run diagnostics and benchmarks on the unit and everything looks great. I haven't seen any issues at all. 

So with all that being said, I'm open to suggestions on what the issue might be. My next thought is to start back at the first available BIOS and work my way back up if it works, but it's honestly a lot of work and I figured I'd ask here and see if anyone has seen something similar. 

Thanks!

[DGroves]

at a guess, i would say that since you moved the board to a diffrent non HP case that the board itself is shorting out by contacting the case somewhere.

remove the board and place paper or some insulator to cover the entire bottom of the case and if possible use plastic feet as spacers from a hardware store to support the board where no or limited support exists (like the x16 gpu slot)

[JakeSnow]

I've done that as well as installing it into the OEM case. Thanks, though!

I also wanted to update a few things-

-It wasn't a BIOS update that I had done before the issue occurred. I had just reset the BIOS to defaults.

-I have done a boot to BIOS with the GPU in, then tested every output to see if it was defaulting to a different output for POST.

-Tested both GPUs in other pci ports with the same result

-Original BIOS I was using was from January '18 so that may be the issue (I found a picture I had taken with the BIOS up). As soon as I can I'll be flashing the BIOS to find out if that fixes the issue. Had my dates mixed up in the original post. There is no BIOS from '19 so I was a year off on each. Didn't have the revisions written down just remembered the months.