Unable to Encrypt Removable Hard Drive on macOS

1. Open Terminal (located in /Applications/Utilities/Terminal.app) and type:

   disktutil cs list
   

2. Then look for the entry of your standard drive, e.g.

   Name:                    Macintosh HD
   

3. Look further down to find out the encryption type:

   Encryption Type:         AES-XTS
   

The latter is what I see when using this command in OS X 10.8.2, so you can assume that AES-XTS stands for the AES-XTS 128-bit encryption used in Mountain Lion.

I expect Lion to use the same encryption model, but I can't verify it because I have no Mac that's running Lion. However, the diskutil command should help you to find the definite answer.

OK, I'm running El Capitan OS X 10.11. The Apple app "Disk Utility" is useless to create encryption, as it typically returns

Operation Failed with status 28

So the fix is to use a little terminal magic. After adding your USB drive to the Mac, run terminal At the command-line prompt, enter:

diskutil list

You will see your main (internal) disk listed, as well as your USB:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            499.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.1 MB   disk0s3
/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           +499.1 GB   disk1
                                 Logical Volume on disk0s2
                                 B0B7A5AA-7D8D-41E3-8565-82E4E1D685D5
                                 Unlocked Encrypted
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *31.5 GB    disk2
   1:             Windows_FAT_32 KINGSTON                31.5 GB    disk2s1

You can spot your external drive by its size -- mine is 32 GB, which, after overhead is around 31.5. In other words, its name is KINGSTON at drive identifier disk2s1. Now, using your USB drive's device identifier in place of disk2s1, pick a suitable name for your new drive. I like thing2:

diskutil eraseDisk JHFS+ thing2 /dev/disk2

About 30 seconds of progress later, you should get:

Finished erase on disk2

Now, open a Finder window, and right click your newly named disk. Pick the option encrypt . In my case, the USB 3.0 encryption took about 12 minutes of effort for 32 GB (45MB/s write). You will know you are done, when the drive activity light goes out.