This guide covers how to safely acquire the official installer and convert it into a bootable ISO image for your 2021 projects. 1. Download the macOS Big Sur Installer
While macOS Big Sur (version 11) officially retired in September 2023 , it received a surprise security update ( version 11.7.11 ) as recently as February 2026 to maintain FaceTime and Messages functionality until 2027. macos big sur iso 2021
hdiutil convert /tmp/BigSur.dmg -format UDTO -o ~/Desktop/BigSur.cdr Rename to .iso: mv ~/Desktop/BigSur.cdr ~/Desktop/BigSur.iso Stack Exchange This guide covers how to safely acquire the