How to view progress when encrypting a disk?
Open up the Terminal and enter the command:

diskutil cs list


Or with APFS starting with 10.13

diskutil apfs list


With APFS the FileVault setup utility also shows health and this status:

fdesetup status


You will see an output listing at least one Logical Volume Group, with a Logical Volume Family and Logical Volume nested below.

There is be a Conversion Status item in the Volume Family entry that will tell you if it's converting to an encrypted volume or not, and for a progress indicator, look under the Logical Volume entry for Size (Total) and Size (Converted) entries, to see how many GB have been converted so far.

For an APFS volume, the output is simpler: there will be an Encryption Progress entry with a percentage as a value (such as 16.0%). This entry will turn into an Encrypted entry, with values Yes or No. As a complement to the value of either field, (Unlocked) may be present if the volume encryption is currently unlocked.

During encryption:

Encryption Status:           Unlocked
    Encryption Type:         AES-XTS
    Conversion Status:       Converting
    Conversion Direction:    forward
    Has Encrypted Extents:   Yes
    Fully Secure:            No
    Passphrase Required:     Yes
 Disk:                       disk6
    Status:                  Online
    Size (Total):            569869340672 B (569.9 GB)
    Size (Converted):        231454277632 B (231.5 GB)
    Revertible:              Yes (unlock and decryption required)
    LV Name:                 Heap
    Volume Name:             Heap
    Content Hint:            Apple_HFS


Encryption completed:

Encryption Status:           Unlocked
    Encryption Type:         AES-XTS
    Conversion Status:       Complete
    Conversion Direction:    -none-
    Has Encrypted Extents:   Yes
    Fully Secure:            Yes
    Passphrase Required:     Yes
 Disk:                       disk6
    Status:                  Online
    Size (Total):            569869340672 B (569.9 GB)
    Size (Converted):        -none-
    Revertible:              Yes (unlock and decryption required)
    LV Name:                 Heap
    Volume Name:             Heap
    Content Hint:            Apple_HFS


The actual Terminal output is nested and longer. Here, only the most important information is listed.



Apparently decrypting an external disk is very slow (or even paused) if you use a laptop which is running on batteries. So always make sure to plug it into mains power when performing this operation.