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:

Disk Utility

Disk Utility

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:

Creating a new image

Creating a new image

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
A mounted image

A mounted image

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:

Entering a password

Entering 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:

Being asked for a password

Being asked for a password

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!


  • Prunk

    Nice topic! I wish I could use .dmg to send files and stuff, but most of my friends use windows, so sad…

    Anyone knows how to burn an .iso image to a cd(and not just burn the .iso file itself) with Disk Utility? Or if there is a way to create .iso images with Disk Utility instead of the .dmg ones?

    • lowell

      > Anyone knows how to burn an .iso image to a cd(and not just burn the .iso file itself) with Disk Utility?

      Yes. You open it and hit ‘Burn’.

    • Gavin

      In Disk Utility, click Images in the top toolbar and then choose burn. Then choose your iso file. Simple !

  • Zy

    Yes, it is one way..(or maybe more) using terminal (aka console).

    Create your desired .dmg file, then use following command input:

    hdiutil convert dmgfilename.dmg -format UDTO -o converted_iso

    • Zy

      Ah it should be not (underscore) converted_iso but converted.iso like follows:

      “hdiutil convert imagefile.dmg -format UDTO -o imagefile.iso”

  • http://lucciotribe.wordpress.com/ tribe

    thats a good question, sometimes i´ve had problemos with converting .dmg to .iso -

  • http://www.thegraphicmac.com/ Jim

    It’s just so much easier to use DMG Canvas (free):
    http://www.macworld.com/article/140134/2009/04/dmgcanvas.html

    • lowell

      that app is for pretty-fying dmg layouts; overkill for making simple dmgs

      • Zy

        Yep :)

  • Geoffrey

    A great app for making both DMG and ISO images AND can do a ton of other stuff is Toast. I know it costs money but it is pretty powerful.

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  • Moon River

    Yes,Disk Utility is a cool choice for mac users.
    Here is another easily guide about “How to Copy DVD Movies with Disk Utility on Mac ?”
    http://www.dvdcopymacosx.com/tutorial/how-to-copy-dvd-with-disk-utility-on-mac.html#129
    :)

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  • Malcolm b

    Enjoyed the article on Disk Utility.
    I am running an iBook G4 with OS 10.5 with Virtual PC for Mac Ver 7.
    Virtual PC wont read CDs in drive D stating it might be corrupted or using a format not compatible with windows.
    I then use Disk Utility to create a .CDR image to my desktop and in Virtual PC under drives it captures the image.
    I have a CD with .WAV files and when attempting to create a .CDR image it gives an error
    unable to create (input/output error)
    Any idea what my problem is?
    Thanking you

  • nicole

    Nice guide, it’s really useful for me. I also found a guide about how to to copy DVD on mac, share it here.

  • Fred Madwid

    Nice but I can`t seem to create a Disk Image without a window telling me that It`s an invalid argument. This Utility seems incredibly complicated to master after having no problems with Nero. With this Mac after creating a new desktop image it`s window refuses drag and drop files. After 2 days I can not figure it out. Please, need some help.

  • bob ama

    “Volume Format: If you have a specific format requirement, feel free to change this – otherwise leave it as the default.”

    But what do the different options mean?

    I can’t find this anywhere in the product documentation or on the web

  • Janet

    I am trying to copy a dvd to my desktop using disk utillity. I keep getting input/output error. What should I do?
    Thanks

  • Presk

    Any idea how to create auto-open window image using disk utility?

    I can do that with Toast 10 however created dmg file is not editable… I cannot add or delete items, not possible to move files around using drag and drop method.

    I have tired to create the toast file with disk auto open window option, but it is the same story, toast file is not editable, so I cannot drag and drop files after mounted.

    I have also tried to change permissions in either toast or dmg file ones created in toast, but nothing happened. It is still read-only…

    One more thing Toast software is set up to write session disk.

  • Laura

    I created a disk utility and did not save my password in the key chain. Now, a couple months later I can’t remember the password. Is there anyway to reset the password?

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  • Nik Simpson

    Nice post!! I want to add one point here about the disk image created by drive utility is that the you can also compress the image image size. For experiment I was using 5 GB of disk drive for image and disk utility provide image of size of 50 MB in return!! I was really impressed with such a nice result. Now image thus formed is portable, take it at any system as it need very short space.

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  • smayer97

    First, I want to point out one capability that is not obvious. Disk Utilities can perform multiple tasks simultaneously. All you have to do is select New from the File menu and you will get another window to set up additional tasks.

    I really like Disk Utilities for its capability, features and functions. But be forewarned. It is a very flaky/buggy application, frequently freezing or quitting for no apparent reason (I am a very experienced IT guy, having supported the Mac since 1986). The versions have become more stable with each new release of OS X but the latest version with OS X.6.8 (Snow Leopard) still has many problems. I cannot speak yet about the Lion version as I do not have that yet.

    Be especially careful with Disk Images. Even after you create one, ALWAYS run Scan for Disk Image afterward to CONFIRM that the disk image is successful (even a Verify Disk Image is NOT enough). I continue to find a number of images that cannot be used because Disk Utilities incorrectly reported they were ok.

  • Mike

    thank you. never thought of those applications for dmg files!

  • Ruggerio

    I have a problem. I created a password protected drive on my Mac Osx Snow Leopard. I can access the drive and read the files but can’t remove the files to my desktop nor from the ‘recycle’ bin A white cross on green back ground appears and it wont let go from my cursor unless i press escape.
    What has gone wrong and how do i get my files back please. Can anyone help me please.

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