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macrumors newbie
Original poster
Oct 13, 2018

I want to run a dual boot with my old setup running Mountain Lion 10.85 and a new setup with
High Sierra. I see I must update the firmware on my mid-2010 MacPro to get High Sierra to work.
If I perform this firmware update and install High Sierra on a second drive will my Mountain Lion
setup still boot or will the firmware update stop it from working?

I can't lose my Mountain Lion setup as I have many music projects and older software installed
and need to keep working on this setup as I switch to the new setup.
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