How to Make a MacOS Big Sur ISO File

Dec 14, 2020 - 16 Comments

How to make MacOS Big Sur ISO File

Some advanced users may wish to create an ISO file of the macOS Big Sur installer file (or MacOS Catalina installer, or MacOS Mojave installers for that matter). These can be useful for installing MacOS into virtual machines like VirtualBox and VMWare, and because the resulting installer is an ISO file it can be helpful for creating an alternative installer media whether on an SD Card, external hard drive, USB flash key, or similar, especially when the typical approach to creating a bootable USB installer drive for MacOS Big Sur is not viable or possible.

Because the macOS installer application is an .app file and do not arrive as a disk image, to create a MacOS ISO file requires a sequence of steps through the command line, or the use of a third party application. For the purposes here, we’ll walk through how you can make a MacOS Big Sur ISO file by using the terminal.

How to Create a MacOS Big Sur ISO File

We will be focusing on making an ISO file for macOS Big Sur, but you can use this same approach for making an ISO file of MacOS Catalina and macOS Mojave too.

  1. Get the MacOS Installer application that you wish to use:
  2. The MacOS installer app should be in the /Applications folder and be labeled as “Install macOS Big” or similar, keep it there and make note of the file name
  3. Next launch the Terminal application by hitting Command+Spacebar and typing “Terminal” and hitting the return key, or by launching it directly from the Utilities folder
  4. First, we must create a temporary disk image:
  5. hdiutil create -o /tmp/MacBigSur -size 12500m -volname MacBigSur -layout SPUD -fs HFS+J

  6. Next, mount the disk image:
  7. hdiutil attach /tmp/MacBigSur.dmg -noverify -mountpoint /Volumes/MacBigSur

  8. Now we’ll use the createinstallmedia utility that is part of the MacOS Installer application to copy the installer files to the disk image you just created:
  9. sudo /Applications/Install\ macOS\ Big\ Sur/Contents/Resources/createinstallmedia --volume /Volumes/MacBigSur --nointeraction

  10. Hit return and enter the admin password to authenticate, let this process complete as it makes the installer that will turn into the ISO. When finished, we then unmount the disk image volume:
  11. hdiutil detach /Volumes/MacBigSur/

  12. Next, we’ll convert the freshly created MacOS Installer disk image file to a CDR / ISO file that will appear on the desktop:
  13. hdiutil convert /tmp/MacBigSur.dmg -format UDTO -o ~/Desktop/MacBigSur.cdr

  14. Finally, we change the file extension from .cdr to .iso:
  15. mv ~/Desktop/MacBigSur.cdr ~/Desktop/BigSur.iso

Assuming you completed the steps properly, you should now have an ISO file called MacBigSur.iso on the Mac desktop. This is basically a variation of converting an installer to ISO as discussed here, which you may have some familiarity with already.

The resulting macOS Big Sur ISO file can now be used to install macOS Big Sur into various virtual machines including VirtualBox and VMWare, and it can also be used to burn to various media including Blu-Ray, SD Cards, external hard drives, and USB Flash drives.

For what it’s worth, you can also convert dmg and cdr files to ISO with Disk Utility too, but the terminal approach to converting dmg to ISO and vice versa with hdiutil is long established and works well, and since you’re already in the command line anyway for working with the createinstallmedia utility the whole process may as well stay in the Terminal.

Obviously this is specifically for if you need to create a MacOS installer ISO file for whatever reason, but it’s worth pointing out that this is not necessary if you simply want to create boot disk USB Installer for macOS Big Sur beta (or final), boot installers for MacOS Catalina, or for MacOS Mojave, all of which are possible by using the createinstallmedia commands and having a USB flash key handy to use as the installer media.

Did this work for you? Do you know of another way to create an ISO file of macOS Big Sur installers, MacOS Catalina installers, or make an ISO of an macOS Mojave installer? Let us know your experiences in the comments below.


Related articles:

Posted by: Paul Horowitz in Mac OS, Tips & Tricks


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  1. Imperfecto says:

    Here is how I did it, all Terminal except the App Store download.

    1: First download macOS Big Sur installer from App Store or if elsewhere and put it in /Applications folder

    2: Make the tmp volume
    sudo hdiutil create -o /tmp/BigSur -size 17600m -volname BigSur -layout SPUD -fs HFS+J

    3: Mounted the disk image
    sudo hdiutil attach /tmp/BigSur.dmg -noverify -mountpoint /Volumes/BigSur

    4: Used the disk image as destination for creating installer using Mac App Store download
    sudo /Applications/Install\ macOS\ Big\ --volume /Volumes/BigSur --nointeraction

    5: Convert the tmp image to cdr file
    hdiutil convert /tmp/BigSur.dmg -format UDTO -o ~/Downloads/BigSur.cdr

    6: Convert the CDR to ISO by renaming it
    mv -v ~/Downloads/BigSur.cdr ~/Downloads/BigSur.iso

    7: Do with the ISO whatever you want, for me it is for creating virtual machine

    8: Eject the original mounted volume or reboot Mac and the volume will eject
    hdiutil eject -force /Volumes/Install\ macOS\ Big\ Sur

    9: Reboot will clear this automatically or you delete the tmp file manually
    sudo rm -rf /tmp/BigSur.dmg

    I did this for a virtual machine, but for other Macs I make a boot flash drive instead.

  2. GregW says:

    Here are the things that worked for me:

    Command not found – only works if the installer image is mounted which you can check in finder and if not there, double click on the ‘Install MacOS Bug Sur’ in the Applications folder.

    The Volume could not be found – make sure there are two minus symbols in front of each argument in step 6 as follows:

    sudo /Applications/Install\ macOS\ Big\ –volume /Volumes/MacBigSur –-nointeraction

  3. Ven says:

    One step missing between 5 and 6, especially if you are trying to do this on Catalina:

    After step 5, your terminal should display something like this:

    sh-3.2# hdiutil attach /tmp/MacBigSur.dmg -noverify -mountpoint /Volumes/MacBigSur
    /dev/disk2 Apple_partition_scheme
    /dev/disk2s1 Apple_partition_map
    /dev/disk2s2 Apple_HFS /Volumes/MacBigSur

    At this point, do the following

    diskutil eraseDisk JHFS+ MacBigSur disk2

  4. Greg says:

    This is the correct sequence:

    hdiutil create -o /tmp/MacBigSur -size 13500m -volname MacBigSur -layout SPUD -fs HFS+J

    hdiutil attach /tmp/MacBigSur.dmg -noverify -mountpoint /Volumes/MacBigSur

    sudo /Applications/Install\ macOS\ Big\ –volume /Volumes/MacBigSur –nointeraction

    sudo hdiutil detach /Volumes/Install\ macOS\ Big\ Sur/ -force

    hdiutil convert /tmp/MacBigSur.dmg -format UDTO -o ~/Desktop/MacBigSur.cdr

    mv ~/Desktop/MacBigSur.cdr ~/Desktop/BigSur.iso

  5. Chuck L says:

    In section (6), I get the command not found after execution. I don’t why. I don’t see anything that is incorrect or missing.

  6. Chuck L says:

    I get the following error on step #6 – Command not found

    I don’t know why!

  7. Chuck L says:

    I get the following error on step #6 – Command not found

    sudo: /Applications/Install macOS Big Sur/Contents/Resources/createinstallmedia: command not found

    I don’t know why!

    • Scooby Doo says:

      The Big Sur installer may have “.app” appended to it’s name.

      Try this:
      sudo /Applications/Install\ macOS\ Big\ –volume /Volumes/MacBigSur –nointeraction

  8. Daniel says:

    You can’t. the installer is static, once you create one, it cant update itself. :-p

    Often times the installer itself isn’t up-to date…There are almost always updates after a fresh install, even when the installer was freshly created.

  9. Sebby says:

    OK, but isn’t this just the same as your previous explainer? And as with that, you can abbreviate the process by creating a CDR image to begin with, instead of converting a dmg into a CDR (see my comment in the linked article).

  10. little sur says:

    How can we create a USB installer that always contains the latest patches without downloading the 12GB file again and again?

    • Daniel says:

      You can’t. the installer is static, once you create one, it cant update itself. :-p

      Often times the installer itself isn’t up-to date…There are almost always updates after a fresh install, even when the installer was freshly created.

  11. Dave G says:

    In section (6) terminal line you left “Beta” within the line.

  12. Daniel says:

    Its simpler to create and mount a disc image then use createinstallmedia the same way you would to create a USB installer pointing the installer at the mounted disk image

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