In OS X it is fairly simple to create a virtual disk image of the type “ISO” by using built-in UNIX tools and the Terminal.app. Unfortunately the Disk Utility.app doesn’t allow the creation of ISO images for all disks (it may work sometime). Here’s what you need to do in order to create an ISO disk image from a CD or DVD on your Mac. Disk imaging is useful for archiving, backing up, duplicating, and restoring systems and can be used to create virtual machines from physical machines and vice versa. Gibbs discusses the concepts.

Disk Utility User Guide

Disk Utility on Mac supports several file system formats:

• Apple File System (APFS): The file system used by macOS 10.13 or later.

• Mac OS Extended: The file system used by macOS 10.12 or earlier.

• MS-DOS (FAT) and ExFAT: File systems that are compatible with Windows.

Apple File System (APFS)

Apple File System (APFS), the default file system for Mac computers using macOS 10.13 or later, features strong encryption, space sharing, snapshots, fast directory sizing, and improved file system fundamentals. While APFS is optimized for the Flash/SSD storage used in recent Mac computers, it can also be used with older systems with traditional hard disk drives (HDD) and external, direct-attached storage. macOS 10.13 or later supports APFS for both bootable and data volumes.

APFS allocates disk space within a container on demand. The disk’s free space is shared and can be allocated to any of the individual volumes in the container as needed. If desired, you can specify reserve and quota sizes for each volume. Each volume uses only part of the overall container, so the available space is the total size of the container, minus the size of all the volumes in the container.

Choose one of the following APFS formats for Mac computers using macOS 10.13 or later.

• APFS: Uses the APFS format.

• APFS (Encrypted): Uses the APFS format and encrypts the volume.

• APFS (Case-sensitive): Uses the APFS format and is case-sensitive to file and folder names. For example, folders named “Homework” and “HOMEWORK” are two different folders.

• APFS (Case-sensitive, Encrypted): Uses the APFS format, is case-sensitive to file and folder names, and encrypts the volume. For example, folders named “Homework” and “HOMEWORK” are two different folders.

You can easily add or delete volumes in APFS containers. Each volume within an APFS container can have its own APFS format—APFS, APFS (Encrypted), APFS (Case-sensitive), or APFS (Case-sensitive, Encrypted).

Mac OS Extended

Choose one of the following Mac OS Extended file system formats for compatibility with Mac computers using macOS 10.12 or earlier.

• Mac OS Extended (Journaled): Uses the Mac format (Journaled HFS Plus) to protect the integrity of the hierarchical file system.

• Mac OS Extended (Journaled, Encrypted): Uses the Mac format, requires a password, and encrypts the partition.

• Mac OS Extended (Case-sensitive, Journaled): Uses the Mac format and is case-sensitive to folder names. For example, folders named “Homework” and “HOMEWORK” are two different folders.

• Mac OS Extended (Case-sensitive, Journaled, Encrypted): Uses the Mac format, is case-sensitive to folder names, requires a password, and encrypts the partition.

Windows-compatible formats

Choose one of the following Windows-compatible file system formats if you are formatting a disk to use with Windows.

• MS-DOS (FAT): Use for Windows volumes that are 32 GB or less.

• ExFAT: Use for Windows volumes that are over 32 GB.

How To Obtain And Use A macOS Disc Image (for OS X 10.7 or later)

• You made a backup copy of the 'Install macOS / OS X' installer package before installing macOS / OS X - click here
• You are about to purchase, download and install macOS / OS X - click here
• You have purchased and downloaded macOS / OS X but have not installed it yet - click here
• You have already purchased, downloaded and installed macOS / OS X - click here
• You have already obtained the macOS / OS X disc image and want to know what to do with the macOS / OS X disc image

Obtain the macOS / OS X Installer - macOS / OS X Not Purchased Yet

1. Go to Macintosh HD > Applications > App Store.
2. Go to Store menu and Sign In if necessary (using the Apple ID you purchased OS X 10.7 or later with).
3. From the 'Store' menu now select 'Search' and search for 'macOS'.
4. Locate the latest version of macOS / OS X e.g. macOS 10.15 Catalina.
5. Click the 'Buy' button
NOTE: If the buy button is not available e.g. it says 'Install', 'Download' or 'Purchased' you have already bought macOS / OS X
6. Purchase and download the macOS / OS X Install installer package to your hard disk (it may take a while as it is 4.3GB+ in size).
7. At the point where the macOS / OS X installer has downloaded it will automatically launch and ask you to continue.
8. Do not click Continue yet.
9. Quit the macOS / OS X installer (yes, quit it).
10. Now go to Obtain The macOS / OS X Disc Image section below

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Obtain the macOS / OS X Installer - macOS / OS X Already Purchased

1. Go to Macintosh HD > Applications > App Store.
2. Go to Store menu and Sign In if necessary (using the Apple ID you purchased OS X 10.7 or later with).
3. Alt-click on the Purchases icon. This will show you a list of all software previously purchased. The 'macOS / OS X' item in the list should have an 'Install' or 'Download' button to the right of it.
NOTE: If the button is greyed out e.g. it says 'Installed' or 'Downloaded' go to the macOS / OS X Already Purchased And Installed section below
4. Click the install or download button for macOS / OS X which will download the macOS / OS X Install installer package to your hard disk (it may take a while as it is 4.3GB+ in size).
5. At the point where the macOS / OS X installer has downloaded it will automatically launch and ask you to continue.
6. Do not click Continue yet.
7. Quit the macOS / OS X installer (yes, quit it).
8. Now go to Obtain The macOS / OS X Disc Image section below

Obtain the macOS / OS X Installer - macOS / OS X Already Purchased And Installed

1. Go to Macintosh HD > Applications and check for an 'Install macOS / OS X' item - if it exists go to the Obtain The macOS / OS X Disc Image section below otherwise continue with these instructions.
NOTE: If you have an additional compatible Mac running Mac OS X 10.6.6 or later that does not have the version of macOS / OS X you want already installed on it you could use that to download the installer again otherwise you will need one of the following external storage devices before continuing with these instructions:
• USB, FireWire or Thunderbolt hard disk
• USB stick
• SDXC card (if your Mac supports it e.g. recent MacBook Pro or Mac mini)
NOTE: The external storage device must be at least 8GB in size and erased/empty/formatted as a GUID Partition before use.
2. Restart your computer using the (Internet) Recovery mode
3. When the macOS / OS X Utilities window appears select the Install macOS / OS X option
4. Choose your external storage device as the destination
5. Let the installation start and watch the installation process carefully (it may take a while as it is 4.3GB+ in size)
6. When the Mac automatically restarts there will be a black screen for a moment - at this point unplug/eject/disconnect the external storage device
7. The Mac may stall for moment but then should boot normally to your desktop or login screen
8. Reattach the external storage device
9. There should be a folder on it called 'macOS / OS X Install Data' that contains a disk image file called 'InstallESD.dmg' - if this is missing, then you probably waited too long to unplug the drive. Go back and start this section again
10. Copy the 'InstallESD.dmg' file to your desktop or preferably to another disk/storage device or even burn it to a DVD so that you have a backup of it.
11. Now choose what you want to do with the macOS / OS X disc image

Obtain The macOS / OS X Disc Image

1. Go to Macintosh HD > Applications and locate the 'Install macOS / OS X version name here' installer package (e.g. 'Install macOS High Sierra') or find your backup copy of it.
2. Right/control click on the 'Install macOS / OS X' installer package and select 'Show Package Contents' from the contextual menu.
3. Navigate to Contents > SharedSupport and locate the 'InstallESD.dmg' file.
4. Copy the 'InstallESD.dmg' file to your desktop or preferably to another disk/storage device or even burn it to a DVD so that you have a backup of it.
5. Close all the windows.
6. You can now use the macOS / OS X disc image (InstallESD.dmg) to:
• Create an External Installation Device.
• Create an Emergency Boot Drive for macOS / OS X / Mac OS X you can use it to boot your computer and repair your hard disk and/or reinstall macOS / OS X / Mac OS X.
• Create an Install DVD (see instructions below).

How To Create An Install DVD From The macOS / OS X Disc Image

1. Obtain the macOS / OS X Disc Image (see instructions above).
2. Go to Macintosh HD > Applications > Utilities > Disk Utility.
3. Click on the 'Burn' button at the top of the Disk Utility window.
4. Navigate to the macOS / OS X disc image ('InstallESD.dmg' file)
5. Select the disc image.
6. Click the 'Burn' button.
7. Insert the blank/unused DVD-R or DVD+R disc.
8. Follow the on screen instructions.

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