Mac Pro 5.1 (4.1) boots into black screen with 7950 in High Sierra

[akamvs]

But this card perfect work at Mac Pro 5.1 2010. At High Sierra, Mojave, Catalina.

 

[SnakeCoils]

I still have a reference Sapphire HD7950 Mac Edition and I can confirm the VGA out from DVI-I issue occour also on this card, it is really an OSX problem. I have flashed some HD7950, 7970 and R9 280x (modifying the native ROMs with Netkas script) and all of them exhibit the same issue. At the time I have also tried to completely bypass the analog native output using some exotic DVI-D to VGA adapters but none of them worked well. I ended in adopting a miniDP to VGA cable too and it solved all my connection needs.

 

[akamvs]

Use the original ROM. It's known that the HD7950 Mac EFI UGA can cause display issue in recent macOS.

That HD7950 Mac EFI UGA only fully compatible to the reference 7950. For any other card, even has reference ports layout, you still have to try your luck.

Black screen

 

[VaZ]

But this card perfect work at Mac Pro 5.1 2010. At High Sierra, Mojave, Catalina.

Your firmware is a decade out of touch..try updating to 144.0.0.0 and maybe things will run smoother √

 

[MIKX]

Same problem with R9 280x and 7970. At Sierra all worked, but in High Sierra black screen. Use DVI, try HDMI, but can't help. At 5770 all worked. Mac Pro 5.1 2012, firmware MP51.0089.B00.

It would greatly help if you told us the GPUs maker eg: Sapphire - Gigabyte - MSI . . . and photos of the output ports - there are different models 7of AMD 7970s and R9 280X cards.

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Example : I have a 7970 ! That tells you so little about this GPU.

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Actually it is an EFI flashed & downvolted Sapphire Dual-X 3gb HD 7970. It is also the 'little brother of the AMD R9 280X. I power this HD 7970 3gb via TWO x Mini 6 pin and ONE 8 pin cable.

EVGA PowerLink with AMD GPUs

Just got the EVGA PowerLink working with my Sapphire Dual-X 7970 3gb. IMPORTANT : The power input sockets on the PowerLink are 2 X 8 Pin female and cannot be changed. The EVGA PowerLink ensures that exact power demanded by a GPU is supplied. That said, one must still supply enough power to...

forums.macrumors.com

This Sapphire Dual-X is my spare, backup GPU.

My everyday GPU is an MSI Armor RX 580 8gb..

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You will get more help if you go to your Macrumors MEMBER PROFILe / SIGNATURE . . .

List the following :

Which Mac OS - & version eg: Mojave 10.14.6 - How much RAM - bootrom version

Look at my SIGNATURE ( SIG ) at the end of this post.

You can edit your SIGNATURE anytime, you can delete it after your problem finds a solution.

PS : The AMD 7950s are getting quite old now. Early last year my HIS IceTurbo HD 7950 EFI flashed. . .just died - out of the blue and despite spending a lot of time on finding the problem I finally got the younger, stronger Sapphire Dual-X 3 gb HD 7970 which - whenever I need to use it, works beautifully

PPS : De-dusting, checking the fans. replacing the GPU Thermal paste are facets of finding the source of the problem.
Doing a fresh, clean install with no `3rd. party apps will often be rewarding.

Good Luck finding the source of the problem.