Creating Bootable ISO from macOS Installer // First Mission

If the freedom of speech is taken away then dumb and silent, we may be led, like sheep, to slaughter.
~ George Washington.

Every once in a while you need something that Apple does not officially hand you, and a bootable macOS installer ISO is exactly that kind of thing. Recently, I’ve been in a situation where I wanted to create a separate portable macOS Mojave1 installer, but I didn’t know how. I tried a few guides, hit dead ends, and finally pieced together these steps from multiple sources until the whole thing worked end to end. This article will assume you are running on macOS system, so if you are on Windows or Linux, bookmark this for later.

Why would you want an ISO at all? Because it is portable. You can mount it, copy it, and store it, and later turn it back into a bootable USB stick or even feed it to a virtual machine. The steps below walk you through the entire process, one command at a time.

📦 Step-by-Step: Building the ISO

1. Download the Installer

First, we download the macOS installer that we want to use from the Apple App Store. I picked the latest one (at this time of writing) which is Mojave. It will land in your /Applications folder, and that location matters, because later steps point directly at it.

2. Export the ISO Path Variable

After that, export an environment variable to hold the output ISO filename that we will be consuming:

export ISO_PATH="mac_installer.iso"

Using a variable here is a small habit with a big payoff: if you ever want a different filename, you change this one line and nothing else.

3. Create the Blank DMG Volume

Then we create a blank dmg volume using the command hdiutil2:

hdiutil create -o "$ISO_PATH.cdr" -size 6g -layout SPUD -fs HFS+J

This command will create an HFS+J filesystem table to store the installer data with the capacity of 6GB. Six gigabytes is enough room for the full installer with headroom to spare, so there is no need to be stingy here.

4. Mount the Volume

Mount the created volume:

hdiutil attach "$ISO_PATH.cdr.dmg" -noverify -mountpoint /Volumes/install_build

If you get a problem while mounting the volume, try to create the folder first using mkdir -p /Volumes/install_build, because the mount point needs to exist before the volume can attach to it.

5. Copy the Installer with createinstallmedia

After that, run the setup to install and copy the files into the volume that we mounted earlier:

sudo "/Applications/Install macOS Mojave.app/Contents/Resources/createinstallmedia" --volume /Volumes/install_build --nointeraction

This is the step that actually copies the installer into the volume, and it is also the slowest, so let it run without touching anything. The --nointeraction flag means it will not stop and ask you questions halfway through. Just a heads up: do not forget the s in /Applications, it is an easy typo and the error message will not be friendly about it.

6. Unmount the Volume

Unmount the volume:

hdiutil detach "/Volumes/Install macOS Mojave"

Note: Check the name first, as the volume name changes after the setup. The installation process specifically overwrites the partition table and volume name, so do not assume it still says install_build.

7. Convert the Image to ISO

We convert the dmg volume to UDTO compatible ISO standard:

hdiutil convert "$ISO_PATH.cdr.dmg" -format UDTO -o "$ISO_PATH"

This is where the raw disk image becomes something that other operating systems and virtual machines can actually recognize.

8. Rename to .iso

Then we rename and remove the .cdr extension:

mv "$ISO_PATH.cdr" "$ISO_PATH"

Just a rename, but it is the step that gives you the familiar .iso file you were after all along.

9. Clean Up

Finally, we delete all the remnants of the procedure:

rm "$ISO_PATH.cdr.dmg"

Nobody likes leftover build artifacts cluttering the disk.

That’s all the steps needed. If you ever completed it without error, then you got yourself a macOS ISO installer.

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🧠 Final Thoughts

This is one of those tasks that looks intimidating until you have done it once, and then it clicks forever: create a blank image, copy the installer into it, convert, rename, clean up. The whole thing is really five real steps hiding behind a nine step list. Keep the ISO somewhere safe, and the next time you need to install Mojave on a fresh machine, or spin it up in a virtual machine, you will already have the fire starter ready.

Hope you guys enjoyed this article!

macOS Mojave is the fifteenth major release of macOS, Apple Inc.’s desktop operating system for Macintosh computers. Mojave was announced at Apple’s Worldwide Developers Conference on June 4, 2018, and was released to the public on September 24, 2018. Wikipedia ↩︎
hdiutil - Manipulate disk images (attach, verify, burn, etc). https://ss64.com/osx/hdiutil.html ↩︎