If you want to install a version of macOS that you previously downloaded from the App Store, then you can just re-download the necessary installer: The next step, is downloading the installer for the version of macOS that you want to run from your external hard drive. Once the process is complete, click ‘Done.’ Disk Utility will now create the partition and setup the drive, which may take a few minutes.Open the ‘Scheme’ dropdown and choose ‘GUID Partition Map.’.In the subsequent window, open the ‘Format’ dropdown and choose ‘Mac OS Extended (Journaled).’.In Disk Utility’s left-hand menu, select your root drive.In the upper-left corner of the Disk Utility app, select the ‘View’ button and make sure ‘Show All Devices’ is selected.Open a new Finder window and navigate to ‘Applications > Utilities.’.Connect the external drive to your Mac.Once you have a suitable hard drive, the first step is preparing this drive as a GUID partition:
In order to install macOS, you’ll need an external drive with at least 12GB of free space.
