Concern over Big Sur firmware update

I have a 2019 iMac with Catalina installed on internal HD. For some legacy app reasons, I am actually running Mojave off an external SSD.
I would like to upgrade to Big Sur on internal HD and do some compatibility testing.
I am concerned that the firmware upgrade from Big Sur might prevent me from continuing to boot Mojave off the external SSD.
Is my concern unfounded? Anyone with any experience on this?

Peter Donlevy

Completely unfounded. Iā€™m actually running Mojave from my internal and Catalina on an external at the moment with the latest firmware update from Monterey on my 2017 iMac. Can even boot Sierra and High Sierra USB externals. Not only have I never experienced a problem, I work with dozens of others that never reported such an issue, either.

If you updated Catalina with the Security Update 2022-002, then you already applied the same, latest Big Sur firmware update.


Good to know - thanks for the info!