How to Make a macOS Big Sur ISO File

There are two ways to install macOS Big Sur on your Mac: using Internet Recovery and using a bootable ISO file. The first method is ideal if you have a strong and stable internet connection. But if you’re in the middle of somewhere where internet connection is not reliable and you need to reinstall macOS, it helps to have a bootable ISO image file in cases of emergency.

Some advanced Mac users might also want to create an ISO image file of the macOS Big Sur installer file or other macOS installers for that matter. They can be handy for installing macOS into virtual machines, such as VirtualBox and VMWare, because the resulting installer is an ISO image. This format can be used for creating an alternative installer media, whether on an SD card, USB drive, or external hard drive. This is especially useful when the typical method of creating a bootable installer drive for macOS is not possible.

Since the macOS installer comes as an .app file and does not arrive as a disk image, you need to follow a series of steps involving commands or third-party apps to create a MacOS ISO file. We’ll walk through the steps to Create macOS Big Sur ISO file and the things you need to watch out for.

Steps to Create macOS Big Sur ISO File

Creating the macOS Big Sur ISO seems like a complicated task, but it is actually very easy as long as you follow the instructions carefully. This guide will focus mainly on how to create macOS Big Sur ISO image file, but you can also use this same approach to create an ISO file for macOS Catalina, Mojave, and other macOS versions.

Before you proceed, make sure you have enough room on your Mac for the installer file that you need to download. For Big Sur, you need at least 35GB – 46GB of free space to download both the installer and the update files. If you have a 128GB SSD, then you’ll need to delete as much junk from your computer as possible. You can use a Mac cleaner, such as Mac repair app, to do a thorough job of sweeping unnecessary files from your computer.

You also need to make sure that you have a stable internet connection to prevent the download from being interrupted. If your internet connection is poor, it might lead to corrupted or incomplete downloading of the installation files, causing your ISO image file not to work.

It is also recommended to reboot your computer before you begin the process to remove any bugs or prevent glitches that could lead to various errors.

Once you’ve got everything in order, follow the instructions below in creating the macOS Big Sur ISO:

1. First, you need to download the macOS installer application that you want to use. For macOS Big Sur, Catalina and Mojave, you can go to the Mac App Store and download the installer applications from there. For older macOS versions that you wish to make an ISO file of, you can check the Purchase tab of the App Store and download your preferred version from there.
2. When you download the MacOS installer app to the /Applications folder, it will be labeled as InstallmacOSBigSur.app or something similar. Save the file in that folder and make note of the file name.
3. Next, open the Terminal application by pressing Command + Spacebar, then typing Terminal in the Spotlight.
4. Hit the Enter key to open the application.
5. You can also launch Terminal directly from the Utilities folder.
6. The next step is to create a temporary disk image by typing in the following command in Terminal, then hit Enter:
   hdiutil create -o /tmp/MacBigSur -size 12500m -volname MacBigSur -layout SPUD -fs HFS+J
7. Then, mount the disk image using the following command:
   hdiutil attach /tmp/MacBigSur.dmg -noverify -mountpoint /Volumes/MacBigSur
8. After this, you can use the Createinstallmedia utility, which is a part of the macOS Installer application, to copy the installer files to the temporary disk image you just created:
   sudo /Applications/Install\ macOS\ Big\ Sur/Contents/Resources/createinstallmedia –volume /Volumes/MacBigSur –nointeraction
9. Press Enter and type in your admin password to authenticate.
10. Wait for this process to be completed because this step is crucial in making the installer that will be converted to ISO.
11. Once done, unmount the disk image volume using this command line and press Enter:
    hdiutil detach /Volumes/MacBigSur/
12. The next step is to convert the freshly created macOS installer disk image file to CDR/ISO file format that will be saved on the desktop:
    hdiutil convert /tmp/MacBigSur.dmg -format UDTO -o ~/Desktop/MacBigSur.cdr
13. The last step is to change the file extension from .cdr to .iso using the following command, then press Enter:
    mv ~/Desktop/MacBigSur.cdr ~/Desktop/BigSur.iso

Once you have completed the steps above, you should now have an ISO image file called MacBigSur.iso on your Mac desktop.

What’s Next?

The macOS Big Sur ISO image file that you just created can now be used to install macOS Big Sur Macs and various virtual machines, such as VirtualBox and VMWare. It can also be burned into various media, such as external hard drives, Blu-Ray, SD Cards, and USB Flash drives.

Although you can also convert DMG and CDR files to ISO using Disk Utility, converting them with hdiutil using Terminal is a lot more convenient and efficient. And since you’re already using commands when working with the createinstallmedia utility, the whole process may as well stay in the Terminal.

More importantly, this method is specifically useful for when you need to create a macOS installer ISO file for whatever reason. However, it is worth pointing out that creating the macOS Big Sur ISO is not necessary if you only want to create a bootable USB Installer for macOS Big Sur beta or other macOS versions. You can do this using the createinstallmedia commands and saving it on the installer media.