Here’s how to get the macOS Big Sur or Catalina installer: When Monterey becomes available to download on 25 October 2021 you should be able to get the installation files via Software Update or the Mac App Store. Whether you want to get the macOS Monterey or the latest beta installation files the process to be similar to that detailed below. This step will depend on whether you want to run a macOS beta, a full version of macOS, or even an older Mac OS X. Now you will see that your external storage has been renamed with the name you choose and it will be ready for you to install the macOS on it – but there is something else you need to do first. Wait while Disk Utility creates the partition and sets up the drive (this can take a few minutes).Choose GUID Partition Map as the Scheme.Choose Mac OS Extended (Journaled) as the Format.Give your drive a name such as “macOS Big Sur” or “USB”.Click on Erase (you need to erase the drive so you can reformat it properly).Select the External drive in the sidebar (the next step won’t work if you only select the volume – in the screenshot above you would need to select the SanDisk, not the NO NAME).Now you will see the root drive in addition to the external volume below it.

Choose Show All Devices from the options. Before this next step – if you are running High Sierra or later you will need to click on the View drop down below the close/minimise buttons.Launch Disk Utility (press Cmd + spacebar and start to type Disk Utility).Connect the external drive to your Mac.For more buying advice, take a look at our guide to the If you want to use the external installation as if you are running it on your Mac we suggest you get the biggest USB C/Thunderbolt 3 drive you can afford. One example is the Samsung X5 Thunderbolt 3 1TB SSD, currently It’s a bit harder to find a Thunderbolt powered SSD, and unfortunately they are a lot more expensive. San Disk USB Type-C drive with 256GB for £30.93 (RRP £59.99). You can get hold of a USB flash stick pretty cheaply, such as this

It’s also worth investing in a USB C or Thunderbolt 3 drive as this will offer the fastest connection.

Make sure that you opt for flash storage because Apple’s APFS (Apple file system) is optimized for SSD and the computer will be sluggish if you attempt to boot from a hard drive. If you want to run macOS on an external drive you will need a few things including a drive with a good amount of storage available. Run macOS on the external drive by pressing Alt/Option at start up on an Intel-powered Mac, or pressing and holding the on switch on an M1 Mac.Get the macOS install files – we’ll show you how.Prepare the external drive for the installation using Disk Utility.

Install big sur from external drive