Give the external hard drive a suitable name and select “Mac OS Extended (Journaled)” as the drive’s format and “GUID Partition Map” as the drive’s partition scheme.Select your external hard drive (not just a single partition).Click View and choose the Show all devices option.Launch Disk Utility (located in Applications -> Utilities).Connect the external hard drive to your Mac.Unless you’re 100% sure that your external hard drive is freshly formatted based on the above-described requirements, we recommend you go through the following preparation steps: More specifically, it must be at least 64 GB large (the more storage space you have, the better) and formatted using the GUID partition scheme and the APFS file system (Mac OS Extended is no longer supported)
To successfully install macOS on an external drive, the drive you want to use must be suitable for the task ahead. Just being curious and wanting to learn something new and potentially useful is the best reason you can have. Of course, you don’t actually need a good reason to install macOS Monterey on an external drive.
For most people, this is Mac's startup drive.By learning how to install macOS on an external hard drive, you can safely test everything macOS 12 Monterey has to offer while preserving your current installation intact.
Performing a clean install of OS X Mountain Lion erases all the data on the target disk. It contains a bootable system with utilities for performing basic disk repair, as well as the ability to reinstall the OS if necessary. This is a hidden partition created by the Mountain Lion installer.
650MB of free space for the Recovery HD partition.A minimum volume size of at least 60GB provides sufficient room for the OS, user data, apps, and some free space. You can squeeze Mountain Lion onto a smaller drive, but there won't be much space left for user data and applications. The target volume must be at least 8GB in size.
You also need a Mac running OS X Snow Leopard (10.6) or later to access the Mac App Store. The OS X Mountain Lion minimum requirements guide lists the Macs that can run Mountain Lion, as well as recommendations beyond the minimum. Screen shot courtesy of Coyote Moon, Inc.