Encrypting non-boot volumes in Mountain Lion
Mountain Lion has made encrypting non-boot volumes without erasing them a much easier process than it was in Lion. In Lion, you needed to go to the command line. In Mountain Lion, encryption is a right-click away. See below the jump for the details.
To encrypt a non-boot volume:
1. Right-click on the volume you want to encrypt
2. Select the Encrypt… command
3. When prompted, enter a password and (optionally) a password hint.
The drive will momentarily disappear from the desktop (to initialize the encryption) then re-appear.
You can monitor the encryption progress by opening Terminal and running the following command:
diskutil corestorage list