Convert macOS installer to ISO

This article explains how to convert .app file type of installer that new versions of macOS come with to an ISO file type that can be used with virtualization software.

As of macOS Mojave, Apple no longer provides a DMG file to install the macOS, it only provides an .app file that needs to be converted to ISO if you want to use it in a virtualization software.

Step-by-step guide

Here are the steps needed to convert the .app file to ISO

  • Download macOS, can be found here

  • Create a virtual disk for the installation media:

hdiutil create -o ./BigSur -size 13000m -layout SPUD -fs HFS+J

The following parameters are customizable, depending on the location and the size of the installer:

  • ./BigSur - location where you want to create the virtual disk
  • 13000m - size in megabytes; size of the Big Sur installer is 12.24 GB, or translated to megabytes, it's 12240 megabytes; 13000m virtual disk size should allow for any additional headspace needed
nex-macpro2019:MacOSApp nex$ pwd
nex-macpro2019:MacOSApp nex$ hdiutil create -o ./BigSur -size 13000m -layout SPUD -fs HFS+J
created: /Volumes/BigDrive/MacOSApp/BigSur.dmg
nex-macpro2019:MacOSApp nex$ ls -alh
total 37729792
drwxr-xr-x   8 nex  staff   256B 31 Dec 15:55 .
drwxrwxr-x  13 nex  staff   416B 30 Dec 20:11 ..
-rw-r--r--@  1 nex  staff    13G 31 Dec 16:06 BigSur.dmg

Please note that all of the commands are ran under the folder/directory /Volumes/BigDrive/MacOSApp

  • Mount the disk:
hdiutil attach ./BigSur.dmg -noverify -mountpoint ./BigSur


nex-macpro2019:MacOSApp nex$ hdiutil attach ./BigSur.dmg -noverify -mountpoint /Volumes/BigSur
/dev/disk5              Apple_partition_scheme
/dev/disk5s1            Apple_partition_map
/dev/disk5s2            Apple_HFS                       /Volumes/BigSur
  • Write the installer to the mount point:
sudo ./Install\ macOS\ Big\ --volume /Volumes/BigSur

This process will take some time, depending on the speed of the disk you're writing to.

nex-macpro2019:MacOSApp nex$ sudo ./Install\ macOS\ Big\ --volume /Volumes/BigSur
Enter PIN for 'Certificate For PIV Authentication (Yubico PIV Authentication)':
Ready to start.
To continue we need to erase the volume at /Volumes/BigSur.
If you wish to continue type (Y) then press return: y
Erasing disk: 0%... 10%... 20%... 30%... 100%
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Making disk bootable...
Install media now available at "/Volumes/Install macOS Big Sur"

At the end of this process, you will have an ISO install image at the chosen destination folder.