Flashed GT 640 would be Mojave equivalent of GT 120.
Yeah. Looks like I'll have to sell an item first, before spending $130 on that. Or, I could install from a MacBook, to an external drive, and then pop that into the Mac Pro to clone it to the machine's main drive. Annoying extra steps, but totally doable. All I'd need to do is pay about $7 for an enclosure for some random disk.
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I believe you make it too complicated. The GT120 EFI should still work, will still display, but may be just no acceleration for the OS UI. You can still upgrade the firmware with GT120 regardless the firmware is for 10.13 or 10.14. Also, you should able to install 10.14 with just the GT120 installed, unless the installer check for a METAL compatible GPU.
But even if METAL compatible GPU is required for installation. A GT120 + METAL supported GPU combo should still works like the current setup. But just don't expect to use the GT120 for anything once boot to desktop (but it should still display).
True... I was planning to install to an external disk, from a MacBook, and clone the finished product to my Mac Pro's main drive, but I could very well see if the GT 120 can just be coupled with my RX 580. After all, all I need to do is use the GT 120 to boot into the installer. Perhaps, after booted into the installer, I can switch the display cables over to the RX 580? (I hope that works.)
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May be more guarantee will be a real single slot card as well.
There is a single-slot GT 640. I want to make sure, though, that it's truly gonna support Metal. As for using it with Windows, that's another story. (I have an AMD card as my main.) Would be cool to try and see if I can keep it in there and use it for Bootcamp, though: I'd simply switch my display over to DVI and go into the bootdisk-selector. Then, I could switch back to the HDMI input going into my main card. Yeah, that would be nice. Although, first, I gotta just see if I can get Mojave onto this from my GT 120. I really cannot wait until the full release.