Software Update shows only software macOS in the App Store. After downloading to your Applications folder, the installer opens automatically.
Select the formatted external drive as the Destination. NOTE: If you get a damaged message when launching, no need to worry. If you want to install OS X Mountain Lion on several Macs, perform a clean install, or boot up a Mac that won't start up, a bootable install media will come in handy. Download 1. OnyX only opens from an administrator account.
