2017 iMac unable to install/boot from Mojave after Installing Monterey

kiwihornplayer wrote:

Before installing Monterey I could run Big Sur with dual boot option of Mojave in order to use 32BIT legacy applications. An external SSD with Mojave booted just fine before Monterey but upon reverting to Big Sur anything below Catalina is locked out as "too old to be used with/ run on this computer". Mojave refuses to boot from external SSD. The installed firmware is still showing as the same that is listed for Mojave thru to Big Sur.

Where do I go from here?

Call Customer Support (800) MY–APPLE (800–692–7753)

or on line https://getsupport.apple.com/

Outside the USA—Contact Apple for support and service by phone

See a list of Apple phone numbers around the world.

https://support.apple.com/en-us/HT201232