MacPro 5,1 mid 2010 High Sierra Firmware Upgrade

[MIB93]

Hi,

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.

 

[hwojtek]

Will work without any trouble.

 

[MIB93]

Will work without any trouble.

Thanks, that's great news.