# Create a blank disk image hdiutil create -o /tmp/Catalina -size 8500m -volname Catalina -layout SPUD -fs HFS+J hdiutil attach /tmp/Catalina.dmg -noverify -mountpoint /Volumes/Catalina Write the installer to the volume sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/Catalina --nointeraction Unmount and convert to ISO hdiutil detach /Volumes/Install\ macOS\ Catalina hdiutil convert /tmp/Catalina.dmg -format UDTO -o ~/Desktop/Catalina.cdr Rename .cdr to .iso mv ~/Desktop/Catalina.cdr ~/Desktop/Catalina.iso