Encrypt external USB thumb drive on MacOS High Sierra
This is an example workflow to encrypt an USB thumbdrive with HSF+ (Journaled) with diskutil using the command line.

Assuming you start with a MS-DOS formatted USB stick.

Step 1: List all currently mounted disks diskutil list:

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *8.1 GB     disk2
   1:                 DOS_FAT_32 MYSTORAGE               8.1 GB     disk2s1 


You see the disk MYSTORAGE has the identifier disk2s1 and is DOS_FAT_32 formatted.

Step 2: Now format the disk disk2 as HSF+ (Journaled):

diskutil eraseDisk JHFS+ "New Storage" GPT disk2


The name of the disk will be "New Storage". At this time it is not yet encrypted. Look at the list of disks diskutil list:

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *8.1 GB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS New Storage             7.7 GB     disk2s2


Step 3: Now you see the "New Storage" partition with identifier disk2s2. Encrypt this partition using:

diskutil cs convert disk2s2 -passphrase


Enter the passphrase when prompted.

If you list the disks now, you also see the encrypted logical volume diskutil list:

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *8.1 GB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage New Storage             7.7 GB     disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3

/dev/disk3 (external, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS New Storage            +7.3 GB     disk3
                                 Logical Volume on disk2s2
                                 8B474F90-34B7-49FE-95E0-E8B260C51CCF
                                 Unlocked Encrypted




If you skip step 3, you can also encrypt the disk using Finder:



Just right-click on the drive and select "Encrypt drive-name".


  Caution: If you choose this alternative approach, the disk gets formatted as APFS encrypted disk!