Você pode usar o seguinte comando para criptografar a partir da linha de comando:
diskutil cs convert /Volumes/your_drive_name_here -passphrase
Você deve obter uma saída semelhante a esta:
computer name:~ username$ diskutil cs convert /Volumes/your_drive_name_here -passphrase
New passphrase for converted volume:
Confirm new passphrase:
Started CoreStorage operation on disk2s1 your_drive_name_here
Resizing disk to fit Core Storage headers
[ - 0%..10%.............................................. ]
Creating Core Storage Logical Volume Group
Attempting to unmount disk2s1
Switching disk2s1 to Core Storage
Waiting for Logical Volume to appear
Mounting Logical Volume
Core Storage LVG UUID: D1EAB2C3-EC21-41DA-AD60-75E1302E247B
Core Storage PV UUID: 991C89E9-A628-408C-AAFF-39A561FCB95C
Core Storage LV UUID: 36483526-6C2C-43FA-A4B7-6F503473F1C2
Core Storage disk: disk3
Finished CoreStorage operation on disk2s1 your_drive_name_here
Encryption in progress; use `diskutil coreStorage list` for status
Eu tenho um post aqui com mais detalhes:
http://derflounder.wordpress.com/2012/01/06/encrypting-10-7-non-boot-volumes-without-erasing-them/