Disk images, or DMG, are a great way to archive files, or move software from one computer to another, because they are digital clones of the physical media they represent. This means you can use a disk image like you would a flash drive or execute a CD without the disk itself.
In this tutorial, I will show you how to write the disk image to a CD or DVD, in order to run software at boot, or just for safekeeping data.
How to Burn a CD or DVD from a Disk Image in OS X
Fire Up That Laser
Prior to the introduction of the App Store, Apple often used disk image files specifically designed for OS X--called DMG files--to install applications. These DMG files are quite flexible within OS X, but are rarely used outside of the typical context of dragging a newly downloaded file to the Applications folder.
The best tool for interacting with DMG is Apple's own Disk Utility. To add a DMG to Disk Utility, simply drag it to the left-side menu of Disk Utility where the other mounted drives are listed (hard drive, SuperDrive, etc.). It will be mounted as a disk and then be available to compress, mount, encrypt or burn.
To burn the disk image to a CD or DVD, simply click the DMG file in the left sidebar, and then click the Burn icon at the top of Disk Utility. You will be prompted to insert your blank media, after which you can then click Burn to confirm that you wish to write the DMG to the disk. Depending on the size of the disk image, and the speed of your writing drive, you will have your freshly minted disk in a matter of minutes.
DMG: More Than Meets the Eye
Apple disk images are also handy because they can store files or applications for transferring to other computers or archiving. To create a new disk image, in Disk Utility simply click the New Image icon at the top, and complete the prompts.
The size selection correlates to sizes of various writable CDs or DVDs so make your selection based on the type of media you intend to store it on. For example, if you are using a 80-minute CD-R disk, choose 663 MB (CD-ROM 80 min); if you're using a DVD-R disk, you'll most likely choose 4.6 GB (DVD-R, DVD-RAM). If you intend to always leave the disk image in digital form, you can choose custom as the size. The format should remain as OS extended (journaled) if you plan to use the disk image with a Mac. Change it to MS-DOS (FAT) if you wish to use it on a Windows PC or Linux.
Finally, you can choose whether or not to encrypt the volume and the partition scheme, though if you choose a disk image size that correlates to a CD or DVD, the CD/DVD partition scheme will automatically be selected.
Once finished, click Create to make the disk image. The DMG will be mounted to your desktop and in Disk Utility and be ready to accept your contents. Be sure to burn the DMG to a CD or DVD, or store it on a flash drive before ejecting the disk image. Otherwise, your contents will be lost.
Tip: If you need to eject (or unmount) a disk image from Disk Utility, simply click and drag it off of the left sidebar of the app; it will poof away just like a discarded Dock app.
Apples to Apples
Since DMG's are designed for OS X, if you intend to take use a disk image on a non-Apple computer, they may prove difficult to work with. For a more widely accepted alternative, look to the ISO filetype, named for the ISO 9660 file system commonly used on CD-ROMs.
ISO disk images, though not specific to use with OS X, work similarly to DMG files. To mount an ISO file, simply drag the ISO to the left sidebar in Disk Utility. Once mounted there, to burn a CD or DVD, click the ISO file to highlight it, and then click the Burn icon at the top. You will be prompted to insert a blank disk, after which you can simply click Burn again to confirm the disk-writing.
ISO files are often used to make LiveCDs of Linux distributions for testing purposes, and burning an ISO through Disk Utility serves that purpose beautifully.
Tip: If you need to open an ISO file beyond burning it to a disk, though, you will need to download an app for modifying archives, such as the free Unarchiver.
An Old Standby
Though there are many ways to store and share files, disk images have stuck around for their ease-of-use and practicality. Now you know about the two types of disk images: DMG and ISO, how to create your own, and how to burn both types to a CD or DVD within OS X.
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