Can't encrypt external hard drive
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.