Creating a disk image is a method often used for system backup, and can essentially clone your hard drive by copying the contents and structure of the disk and saving them exactly as they are on your computer. Using Mac’s Disk Utility, you can create a password-protected and fully encrypted disk image for secure system backup and disk cloning. Use the steps below to create a protected disk image for your Mac’s hard drive.
Open the Disk Utility
To get started with creating a protected disk image, you will need to find and open the Disk Utility on your Mac. The Disk Utility can be found by opening your Utilities folder (inside the Applications folder) and selecting the “Disk Utility” program. The icon for Disk Utility includes a hard drive and a doctor’s stethoscope.
Once you’ve opened Disk Utility, just look toward the top of the program window, just below where it says “Disk Utility.” You should see a button that says “New Image” with a + sign next to it. Click this button to begin the disk image creation process.
Enter the Information for Your New Disk Image
Before your Mac will actually start preparing a disk image of your hard drive, you will need to decide where on your hard drive to save the image. Depending on the contents of your hard drive, there might actually not be enough space on the drive to save the disk image.
Keep in mind that the disk image, to capture everything on your hard drive, needs to be as large as all of the files on your drive combined. So if you have used 300 gigabytes of a 500 gigabyte hard drive, the disk image needs to be 300 gigabytes. Needless to say, you might need to save it on an external hard drive. (To find out how much of your disk’s storage is currently in use, select the Apple menu, click “About This Mac,” and select the “Storage” tab.) You will also need to decide how large you want your disk image to be before it is created, so you will need the have the storage information anyway. Your disk image will occupy the chosen amount of hard drive space right away, so if you say you need it to be 300 gigabytes, it will immediately fill up that space to accommodate the backup.
In these early steps, you will also have to enter: a name for your disk image; the format for the disk image’s file system—for which you will need to choose “Mac OS X Extended (Journaled)”; the level of encryption you want for the disk image (you can choose between 128-bit and 256-bit AES); partitions for the disk image (select “Single Partition” here); and a format for the disk image itself (for which you will select “read/write disk image”).
Once you have entered all of this information as detailed above, you are ready for your computer to actually go ahead and create the disk image. Check your entries to make sure they are correct, and then click the “Create” button to start the process.
Decide on a Secure Password
Once you click create, your Mac will request that you create an encryption password for the image. Think carefully when choosing your password, since it will grant access to your entire hard drive backup. You want your password to be something that you won’t forget, but also something that won’t be easily guessed or hacked. Helpfully, Apple includes a monitor in this dialogue box that will tell you whether your password is strong, weak, or in between. (If you need tips creating a great password, read our full article on passwords.)
Once you’ve chosen and verified your password, click “OK.” Your Mac will go ahead and create, encrypt, format, and mount the disk image. This process can vary in length depending on the size of the disk image that needs to be created, as well as upon which level of encryption you chose earlier. (128-bit AES leads to quicker disk image creation, but won’t be as secure, while 256-bit will add to both security and disk image creation time.)
In the future, you will be able to add new files your disk image (rather than creating a whole new one). In fact, you can even enlarge or shrink the size of your disk image after the fact—perhaps because your disk image isn’t filling the container and is therefore wasting space on your internal or external hard drive, or maybe because you need to add more files to an already full container. And of course, you will also be able to restore from your disk image at any point in the future.