Encrypt An External Disk Volume
# diskutil cs convert /Volumes/Spare -passphrase Or split into two steps:
# diskutil cs convert /Volumes/Spare
# diskutil cs encryptVolume /Volumes/Spare -passphrase To check the status of the encryption process:
# diskutil cs info /Volumes/Spare Obviously the volumename 'Spare' should be replaced by your volume in question...