Install Old Macos

Installing macOS High Sierra and Catalina and older versions

I was looking for information on how to install Catalina on a system that had High Sierra. All the blog posts and articles I can find and online talk about downloading it from the Mac App Store. Even Apple’s official support page implies that you can download from the Mac App Store.

The Easy Way

If you read closely, you’ll see that Apple’s official instructions say “Safari uses these links to find the old installers in the App Store”. What isn’t said, but is true, is that if you use any browser other than Safari, it won’t work. The Easy Way, then, is:

  1. Use Safari
  2. Follow the instructions at Apple Support

I use Firefox for lots of good reasons. When I click the link for Catalina in Firefox, Firefox prompts me to open the Mac App Store. It looks right. But the Mac App Store tells me that the connection to the store has timed out. If I use Safari to click that link, however, the Mac App Store opens, and it actually works.

The Hard Way

Since it didn’t occur to me to use Safari on those links, I ended up finding a harder, but effective way to do this. It is a bit technical, but it downloads authentic macOS from Apple, not some dodgy crap from an ad-sponsored SEO-optimised, clickbait linkfarm. I found a lot of clickbait web sites with broken links and a zillion ads. Not useful.


Before you can be successful at this mechanism, you need to understand that it’s going to involve using the Terminal, Python at the command line, and downloading code from GitHub. If you can’t do those things, you’re gonna have to learn before you can follow these instructions.

Step 1: Download a Python Script

Download from the OSX-KVM project on GitHub. I’m not going to tell you how to use GitHub if you don’t know how. But I recommend clicking on the raw link and then saving that to a file named in a location you can find on your Mac.

Step 2: Make Sure You Have Python 3

Now, this is a Python 3 script, and if you’re running an old version of High Sierra or Sierra or something, there’s a decent chance you don’t have Python 3. We have to make sure you have Python 3 before we proceed.

  1. Open a Terminal.

  2. Type python3 -V and press enter.

    a. If you see something like Python 3.6 or similar, you’re fine

    b. If you see something like -bash: python3: command not found, then you do not have Python 3. You will not be able to proceed until you install Python 3. I talk about that down below.

Step 3: Run the Script

You need to know where that file is on your computer. A lot of the time, it will be in your Downloads folder. If it isn’t, you have to find it. I can’t tell you where it might be. I’m going to assume it’s in your Downloads folder.

Type the following commands in your Terminal window:

cd Downloads

You will be presented with a menu like the following:

1. High Sierra (10.13)
2. Mojave (10.14)
3. Catalina (10.15)
4. Big Sur (11.6) - RECOMMENDED
5. Monterey (latest)

Choose a product to download (1-5):

Simply type the number of the version you want, and press enter. The download is a few hundred megabytes. Not more than 1 gigabyte. Let it download.

Step 4: Open the DMG

Disk images are a convenient way of distributing a lot of files, kinda like a zip file. You will have a file named BaseSystem.dmg in whatever directory you’re in. (Above, I assumed we were in your Downloads folder).

I’m lazy. I type open BaseSystem.dmg and it opens.

Base System Installer

Optional Step 4½

Maybe you did these instructions on a more modern Mac or on a Mac that is not the one you needed to upgrade. If that’s the case, copy this BaseSystem.dmg file to the Mac you want to upgrade. It’s all that you need.

Step 5: Double-click the Installer

On the system you want to upgrade, click the Install macOS icon. It will run.

Note: The system that you are upgrading must have a working Internet connection during this upgrade. It will download all the bits of macOS that it needs. That’s a total download of several gigabytes at a minimum. It will take as long as it takes.


The software will install and reboot and install and reboot over and over. It eventually stops rebooting and you have a working system.

Once you’ve got it installed, be sure to install Homebrew.

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