Creating Disk Images with Disk Utility

Disk Utility is an excellent OS X utility for managing hard drives and removable storage. If you’ve ever installed OS X, wiped a hard drive clean, or needed to re-format a USB stick, there’s a good chance that you’ll be familiar with the app. Whilst managing disk images is undoubtedly Disk Utility’s forté, it can also be used to good effect for creating images.
This how-to will walk you through how simple this process is. We’ll illustrate how to create a simple disk image for storing files, a few of the uses that images can have, and also investigate how images can be encrypted to keep your files secure.

Creating a New Image

When opening Disk Utility (found in the Applications > Utilities folder), you’ll be presented with a screen which looks similar to the following:

Don’t be intimidated by the impressive technical jargon – what we’re going to do is very straight forward! Disk Utility has the ability to perform all manner of complex operations on a hard drive, but today we’re only concerned with one button on the toolbar: ‘New Image’.
Clicking ‘New Image’ will bring up a dialog with several different options – essentially the recipe for what will be included in the new storage space we’re creating:

At first glance this can seem a little confusing. Here is a simple explanation of how to handle each different option:
  • Volume Name: This is simply the name which will be given to the image when mounted
  • Volume Size: A disk image needs to be a specific size. A range of popular values (e.g. for a CD or DVD) are already available to choose, or you can enter a custom size.
  • Volume Format: If you have a specific format requirement, feel free to change this – otherwise leave it as the default.
  • Encryption: You can choose between two types of encryption, and this option is covered in more detail below
  • Partitions: Here you can select whether you’d like to make a DVD/CD image, regular image, or one which can be booted by OS X
  • Image Format: Generally stick with the default here, unless you have advanced requirements
 After filling in all the various options, click ‘Create’. The time it takes to make the image will depend upon the size selected, though it should be a fairly quick process. When Disk Utility completes it will automatically mount the new image on your desktop, open and ready to have files added.
When you have added the desired files, dragging the mounted image to the Trash will ‘Eject’ it. The disk image can then be moved around or copied between computers safely.

Dealing with Encryption and Security

One of the most useful features of a disk image is the ability to create a secure area for storing confidential information. They can be encrypted with either 128-bit or 256-bit security (which is stronger, but takes longer to encrypt files). If this option is selected when creating an image you will be presented with the following dialog to enter a password:

If ‘Remember password in my keychain’ is selected, you will never be asked for it when using the image in your current user account. Moving the image to another computer (or attempted access by another user) would require the password to be entered. I prefer to leave this unchecked, as it means that the files contained within the image are extra-safe.
When opening the image (having not checked ‘Remember…’) you’ll be asked to enter the password previously specified:

Disk Image Uses

A number of functions exist for disk images, and – despite what many Mac users think – they aren’t just used when downloading a new application. For me, the level of security they provide is essential when storing information on a USB stick. It means that you don’t need to worry about it being lost or stolen (unless the thief in question has the computing power of a government agency!).
Another interesting use is to create a disk image of a physical CD or DVD. This means it’s possible to mount the CD from an image stored on your hard drive – without ever again needing to find the physical disk.
Disk Utility is an incredibly powerful tool and we have simply scratched the surface of what it can do. If there are any features of the app you would like to see explored in the future, please let me know in the comments!

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