Title basically.
If I swap out GPUs, the system works fine. So it seems to be the GPU at fault. This GPU has a switch for overclocking (two sets of BIOS). I switched it while the system was powered off multiple times testing things and this is the situation I’m in now.
I’m not sure if the symptom (only booting to a black screen with a _ and never going on to the splash screen) sounds like it’s caused by what I think it’s caused by.
Is that from cold boot?
Does reset button cure the issue?
Yes.
No.
What GPU?
I’ve had similar symptom with a Sapphire Nitro RX 480 with a dual BIOS. Silent BIOS booted. Turbo BIOS wouldn’t. Turned out the motherboard UEFI implementation was old and not fully compatible so I disabled the UEFI booting I think or at least played with the legacy options and it would boot to Windows but no splash screen or BIOS prompt.
Just a possibility a BIOS option has changed. Otherwise it still may be possible to flash the GPU BIOS if you can get it to boot in another system as a 2nd card? I could do it easily with RX 470/480s but I’ve no experience with other AMD GPUs.
That is so strange. I have been using this card for months and I haven’t made a single BIOS change in that time. And suddenly this happens.
Another Radeon Pro Duo user said this is normal behavior for him. He has to put a Vega 56 card in his 2nd PCI-e slot and plug the display there to see the boot sequence succeed.
Really weird stuff tbh. Thanks, I’ll definitely try his method (a second GPU), then if that doesn’t work, I’ll try a different system, and if that doesn’t work, I don’t know what I’ll do.
If you have not done any changes in the UEFI, have you done any changes to the GPU BIOS?
If not, go to AMD’s website and follow their driver removal tutorial, with utility provided. Then reinstall drivers. You may just have a corrupted driver.
Other than flipping the GPU BIOS switch, I have not.
Hmm, I installed an RX 560 to test and it boots fine. No issues at all. Switching back to the Pro Duo and the same issue occurs.
Uninstall the drivers with the utility and reinstall.
If that fixes it I will be amazed.
Here’s to hoping. 
May not hurt to clear CMOS…
I mean, if it is the drivers, then I don’t understand why it wouldn’t seem to POST.
I’ll try that next if this doesn’t work.
I have Vega 64s. On cold boot, occasionally, on my triple monitor set up, my center screen is black and two side monitors are white with coloured lines. Does not boot to windows.
And GPU2 is pinned at full out. This is driver failure.
Neat.
I uninstalled the drivers and cleared the CMOS by unplugging the battery and disconnecting power for 60 seconds.
I used AMD’s instructions then their Cleaner Utility.
The Radeon Pro Duo still only shows the _ screen. 
So only black screen on both BIOS settings? You can try flashing the BIOS on the left. If that does not work, might have to send in for repair.
What GPU do you use that boots successfully? Does it use same power connectors?
Yes. I will try, but I don’t think they do BIOS repair. :c
RX 560
Everything is identical. Same Power supply connectors (I’ve tested all 3). Same DisplayPort cable. Same PCI-e slot (since there’s only 1). Same Motherboard BIOS settings.
Massive difference in power draw. You can not rule out the PSU…
That’s a fair point.
That’s the PSU.
The system has a RyZen 1700 CPU which has a 65W TDP. The Radeon Pro Duo has a 375W TDP. I’m not overclocking either. They’re both water cooled, so they’re both more efficient than with air cooling because they stay sub-30C on idle and sub-60C on load (from what I’ve tested).
I should be using about 75% of the total available power output of this PSU, so I wouldn’t think this is the problem.
I own one of these as well:
I’ll try that I guess.