How to install macOS High Sierra on an external USB drive

The public beta of macOS High Sierra is available for download and you can test the new and exciting OS update on your Mac. Install it on a USB drive to leave your Mac running macOS Sierra.

Beta software may or may contain bugs, may have unfinished features, may have features not yet included, and included features may be left out of the final version.

For these reasons you might not want to put beta software on an iMac or MacBook that you rely on for everyday work. Although Apple public beta software is usually pretty stable and feature complete, it is still possible for something to go wrong.

However, beta software gives you a glimpse of things to come and macOS High Sierra has some exciting features. Who wouldn’t want to try it?

There are two options when installing beta software and one is to use a computer that isn’t important. If you two or three iMacs and MacBook, and one isn’t essential, the new OS could be installed directly on the disk.

It would be nice to have spare Macs, but most people do not. I plugged in an external USB disk drive and installed High Sierra on that instead of the MacBook’s internal drive.

So I now have macOS Sierra running on the internal drive, but have the option to boot from the external drive running macOS High Sierra. Here’s how to set it up.

Sign up for betas

Apple has a website where beta software is made available at beta/apple.com and you can sign up and sign in. Just use your Apple ID, the same one you use for iCloud, iTunes and other Apple services.

It provides access to more than just macOS betas, but that is the one we are interested in here. Select the macOS tab and click Enroll Device at the top of the page.

There is a small app to download and install, the macOS Public Beta Access Utility, and this is like a bare bones email app. It is used for sending feedback to Apple, such as problems encountered so Apple can fix them before macOS is finalised and released. This is beta software, so expect bugs!

Download macOS High Sierra

On the macOS beta page is a link to view macOS High Sierra in the Mac App Store. This cannot be found in the normal way because it is hidden. Click the link, the store opens and you can download macOS.

It is a 5GB download, so make sure you do this on a fast internet connection or it will take all day!

Install macOS High Sierra

The macOS High Sierra installer looks unchanged from previous versions of macOS and OS X before that.

The installer automatically runs once the download is complete, but you can quit and install it later. It is an app in the Applications folder called Install macOS High Sierra beta.app.

Click Continue on the opening screen and click through the legal terms and conditions.

Choose the disk to install macOS

As with previous macOS installers, the default is to upgrade the current macOS installation on the Mac’s internal disk. Most people should just click Install and proceed with the install.

If you want to install it on a different disk or partition, as I do, click Show All Disks.

All the available disks and partitions is shown. Click the one you want and then click the Install button.

In my case I want to install macOS to an external USB disk drive called Disk 1. It has a backup of macOS Sierra and High Sierra will upgrade it.

Boot the Mac from an external drive

During installation the Mac will restart several times and this is normal. What you need to do each time is to make sure the Mac starts up with the right disk.

There is nothing to do when installing macOS to the internal disk and this is the default startup disk. If you want to start from an external drive, as I do, installing macOS on a USB drive, hold down the Option key as soon as the screen turns gray. It’s a second or two after turning on the power or restarting.

A list of bootable disks and partitions is displayed and the one to start the Mac with is selected with the left and right arrow keys and then Enter to finish.

If external USB drives do not appear, unplug them, wait 10 seconds and then plug them back in. The Mac will then detect them and add their icons to the screen.

When installation is complete, you can start your Mac with the Option key held down to choose which disk/partition to boot from.

Related: What to do before installing OS X Yosemite It applies to macOS High Sierra too - things haven't changed much!