Mac Pro 5.1 Dual GPU Problem - Vega 56 and ATI Radeon 5770

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Level 1

4 points

Mac Pro 5.1 Dual GPU Problem - Vega 56 and ATI Radeon 5770

Hi everyone!

I bought some time ago Mac Pro 5.1 with original ATI 5770 GPU. I did immediately the CPU, RAM, SSD upgrade and also switched the factory GPU to Vega RX 56 8GB to be able to update the machine to Mac OS Mojave and flashed it to have the boot screen.

The main reason for me to buy this Mac was to launch the original Final Cut Studio 3 and Logic Studio boxes that I bought years ago as a "decoration". I read that an app called Retroactive should be able to open both software on Mojave but unluckily it's working only with Final Cut and Logic Pro apps with all the other stuff like Compressor or DVD Studio.

So I came with an idea to install an older OSX version on a separate drive in my Mac. I did the Pixlas mod to power the Vega 56 directly with the PSU (earlier I used two power cords to use the original power slots in the motherboard). The pixlas mod went successfully, so it was time to plug the other GPU to be able to run the older OSX. I used the factory GPU that I got with the computer (ATI Radeon HD 5770) and powered it with this one cable to the port in the motherboard... and the Mac stopped working.

With the combination of two GPUs, Mac starts, does the sound, both GPUs are running, but there is no image on the screen. Everything works fine again as soon as I unplug the ATI card power cable from the motherboard. Then is everything okay but that's not the effect I wanted to get.

With this only card (Vega 56) I installed OS X Lion on another drive without any problems. I can even launch this system but it shows 15MB of VRAM in the tech specs on Lion (lack of drivers, I suppose) that casues unability to install any desired software becasuse it requires min. 1GB of VRAM.

Please help me and tell me what can I do to be able to use both GPUs. I think I tried everything possible. I changed the PCI-E slots of the GPUs, tried different cables, plugged them, unplugged them and it did nothing.

Earlier Mac models

Posted on Apr 19, 2026 10:00 AM

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Answer 1

Apr 26, 2026 7:43 AM in response to blabud

Drivers for Vega 56 first appeared in MacOS 10.13.

Using that card with older macOS is unlikely to yield success.

When you over-draw the Mac Pro power supply, it typically does an emergency power-down to save itself.

Most users find a way to use one card or another, because two (as different as those two) active at once does not always work so well. Do you need to drive a whole lot of screens at once? If not, choose ONE card.

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Answer 2

kaz-k

Level 9

51,674 points

Apr 25, 2026 7:26 PM in response to blabud

Hi,

The Mac Pro 5,1(2010-2012) officially supports GPUs with a maximum power draw of around 225W–300W using internal PCIe AUX power cables. The board provides 75W via the PCIe slot, and two 6-pin connectors provide an additional 75W each, but most users can safely run 250W–300W cards by using 6-pin to 8-pin adapters.

I think power connectors for GPU on motherboard are not able to supply good enough power to the two GPUs at the same time. You would need another power source for GPUs.

This is a different story, though I used second optical drive power cable with SATA power cable adapter for the fans.

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