Securing Data on Your Mac with an Encrypted Volume
If you have some data on your Mac that you want to protect, but you don’t want to go to the bother of encrypting your entire user directory, you have another option. You can create an encrypted disk image, which looks like a disk volume when you open it. An encrypted volume looks very much like the volumes typically created when you download an application program. It even has the same .dmg extension, but you will need a password to open it.
Go to the Utilities folder in your Applications folder and open Disk Utility. Choose File→New→New Blank Image.
The New Blank Image dialog appears.
Pick a size for your volume that’s large enough for everything you plan to store in it, with room to spare.
If you keep it below 700MB, the encrypted volume will fit on a CD-R if you choose to back it up that way.
Give the volume a name, select AES-128 Encryption, and click Create. When prompted, enter a password.
If you click the small Key icon, Password Assistant appears to offer suggestions.
If you don’t want OS X to store the password in Keychain, deselect the Remember Password check box.
You’ve now encrypted your data.