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Mac Pro intel with Westmere processor(s) must be a 2010 or later model, unless upgraded to a Mac Pro 5.1. In either case, it can run MacOS 10.11 El Capitan without further modification, and likely up to 10.13 High Sierra. If you install a Metal-capable graphics card, it can even run 10.14 Mojave.
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Thanks for clarifying that. Somehow I got confused… :)
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Next higher is a simple x4 PCIe card (under US$30 for the empty card) that could provide nominal 1500 M Bytes/sec in your Mac Pro 5,1 in a PCIe2 x4 slot. Installation is slightly more complex, because you must apply a heat transfer pad to the SSD stick and bolt on a heatsink (included in the kits).
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Will I be able to use an SSD drive as a boot drive if I install it on a PCIe card instead of having it in the first drive bay of the Mac Pro?