The Disk Utility program in Mac OS X enables you to create disk images (.dmg), similar to those you encounter when you install software on your Mac. When you double-click on this type of file, your computer mounts it as though it were a DVD or hard drive. In fact, you can think of mounted disk images as virtual drives. You can also add password protection to disk images when you create them. Without the correct password, it’s almost impossible to read the contents of an encrypted disk image, so it’s a great way to transfer data securely via flash drive, CD, or DVD or travel with files you want to keep confidential.

It's important to note, however, that the Macintosh Disk Utility  does not replace the use of FileVault for storing data more permanently on a dekstop or laptop Mac. Remember, too, that Level 1 data must be permanently removed from portable devices if it is not covered by a current and approved business need.

Creating a Disk Image

To create a disk image from a folder on your Mac, first open Disk Utility. You’ll find it in the Utilities folder inside the Applications folder.

1. From the File menu, select New, then New Disk Image from Folder (or type Command-shift-N).
2. Navigate to the desired folder, then click Image.
3. A pop-up menu prompts you to choose a name and save location for your disk image. It also includes two pull-down options: Image Format and Encryption.
4. If you’re going to transfer your disk image (for example, via a flash drive), select Compressed under the Image Format pull-down.
5. To add password protection, select 128-bit or 256-bit AES encryption under the Encryption pull-down. 128-bit encryption meets HSU's security requirements and is faster to create than 256-bit.
6. Click Save. Disk Utility will begin to create the new disk image with the name and preferences you’ve specified.
7. If you chose to add encryption, a password pop-up appears. Enter and verify a strong password. Disk Utility evaluates the password’s strength and gives you the option of remembering the password in your Mac’s keychain. If you lose the password, you'll probably never be able to access the data again.
   

ITS has also prepared a short video tutorial to illustrate the process of creating a Disk Image.

Now you can transfer the disk image easily and securely via USB drive or other means. The only way to access the data inside the disk image is by entering the correct password.

Note that Disk Utility can make disk images from folders, but not from individual files. If you want to create a disk image for a single file, just create and name a new folder, place your file inside, and make a new disk image from that folder using the steps above.