How to upgrade Graphics card for Mac Pro running MacOS High Sierra

No, we can't.

The graphics cards in the Mac Pro 6,1 dark cylinder from 2013 (there are two that CAN work together with software that recognizes them) are NOT replaceable. They are custom form-factor cards, and no one has chosen the develop aftermarket cards for them.

An External PCIe card enclosure might be an interesting idea, but there are none that work with that model's ThunderBolt-2.

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But if all you want is Metal support, the cards you have support Metal innately, and MacOS through MacOS 12 Monterey.

the Mac Pro 2013 model looks like this:

if you have a 42 lb silver tower 2012 or slightly older model, there are many more options...

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