[SOLVED] How to create a MacOS High Sierra boot disk on USB thumb drive

How to create a macOS High Sierra boot disk…

TL;DR — Just run the installer and select the external dive. Scroll down for screen grabs

If, like me, you’ve been trying to create a boot disk for your MacBook to run High Sierra from an external drive, you’ve probably run into a few issues.

Following the instructions on the Apple website, here, I had this error on the command line…

/Applications/Install macOS High Sierra.app does not appear to be a valid OS installer application.


Several other articles I read pointed me at other tools to download but they didn’t work either. If you’ve tried them you might have come across these errors…

How to create a macOS High Sierra boot disk
Invalid macOS High Sierra Installer error
Invalid macOS Installer

The selected item does not appear to be a valid macOS installer.

It is lacking the file ""Contents/SharedSupport/InstallESD,dmg


One of the other tools I tried also failed.

How to create a macOS High Sierra boot disk
Invalid macOS High Sierra Installer error
The disk could not be created because of an error: An error occured: -10006. Finder got an error: Can’t set alias "Install macOS High Sierra:Install macOS High Sierra.app" to {425, 76}.


The Solution…

…I found was so simple and obvious I’m actually embarrassed to publish it here. I’m going to anyway because I’m sure others out there will be running into this problem and desperately searching for the answer.

Step 1: Backup your hard disk…

…because if you make a mistake you will lose your data. If you’re not sure how to do this, ask a local Mac expert for help

Step 2: Prepare your USB thumb drive

Use the Disk Utility app to format your external drive. You can find it under Applications | Utilities | Disk Utility.

WARNING: Be careful to make sure you select your external drive or you will lose everything

Step 3: Run the macOS High Sierra Installer

Make you’re your USB thumb drive is connected then download and run the macOS High Sierra installer from the App Store. You should be greeted with a welcome screen like this one:

macOS High Sierra Installer welcome screen
macOS High Sierra Installer welcome screen

Clicking the ‘Continue’ button will take you to the Terms and Conditions screen, thus:

macOS High Sierra T&C
macOS High Sierra T&C

Read it and if you’re OK with it, click through to the next screen…

Show All Disks button -- macOS High Sierra Installer
Show All Disks button — macOS High Sierra Installer

When you get to this step, the installer gives you the option to choose your destination disk. Look for the button indicated by the BLUE ARROW that says ‘Show all disks’. If it’s not there, STOP, quit the installer, check your thumb drive is connected and working properly then re-start the installer.

Click the button and you should see a screen something like this one:

Select your thumb drive -- macOS High Sierra Installer
Select your thumb drive — macOS High Sierra Installer

Look through the list of dives presented to you and select your thumb drive. I called mine ‘BootDisk’ when I formatted it to make it easier to recognise.

WARNING: Make sure you select your thumb drive or you will overwrite the OS on your MacBook

Clicking the install button will start the installation process. For me it too several hours as the download was over 5.5GB and the installer has a lot of work to do. Your MacBook will reboot itself several times through out the process and give some worrying beeps too. Just be patient, let it do it’s thing and wait for the High Sierra welcome screen to appear once it’s completed.


As I said earlier, I can’t believe how incredibly simple and obvious this solution was. Creating an external boot disk turned out to be very, very simple and works perfectly:

To run High Sierra, simply plug in the thumb drive then reboot/turn on your macBook.

To run with your legacy OS (in my case it’s Mavericks), shut down, remove the thumb drive and restart.


BTW, in case you where wondering…

Why do I want to boot High Sierra from an external drive rather than upgrade from Mavericks?

Photoshop and Lightroom. I run a photography business and when I bought my MacBook I spent a small fortune upgrading from Photoshop CS3 on Windows to Photoshop CS5 on my MacBook. Photoshop CS5 does everything I need and a whole lot more besides so there is no business case for migrating to Adobe’s license subscription model. Everything still works perfectly fine under Mavericks.

Neither of these tools are supported on High Sierra as they’re beyond end of life. They may still work under High Sierra but I’m not prepared to take the risk.

I also need the latest version of XCODE to develop iOS apps. This isn’t available for Mavericks, creating the need to migrate to High Sierra.

Booting from an external USB drive was a good workaround allowing me to preserve the integrity of my legacy system and take advantage of the latest development tools. It was an easy proof of concept and now that everything is working fine at some point I might repartition my internal disk and move to a dual boot scenario instead.

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