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.

On OS X 10.11.1 the output diskutil cs list shows encryption progress as:

…

+-- Logical Volume Group 19B060CE-52A6-4102-9F3D-E6108BD91316
    =========================================================
    Name:         My harddrive
    Status:       Online
    Size:         499113885696 B (499.1 GB)
    Free Space:   18972672 B (19.0 MB)
    |
    +-< Physical Volume 3F2AE785-3A81-45E2-9A3B-93BA762C0AD2
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk5s2
    |   Status:   Online
    |   Size:     499113885696 B (499.1 GB)
    |
    +-> Logical Volume Family E4B845E2-2294-4F26-89B5-25ADE02747D2
        ----------------------------------------------------------
        Encryption Type:         AES-XTS
        Encryption Status:       Unlocked
        Conversion Status:       Converting (forward)
        High Level Queries:      Not Fully Secure
        |                        Passphrase Required
        |                        Accepts New Users
        |                        Has Visible Users
        |                        Has Volume Key
        |
        +-> Logical Volume 58EA9B33-CD1A-49F7-A596-E2C5ED6E24B5
            ---------------------------------------------------
            Disk:                  disk6
            Status:                Online
            Size (Total):          498742591488 B (498.7 GB)
            Conversion Progress:   2%
            Revertible:            Yes (unlock and decryption required)
            LV Name:               My harddrive
            Volume Name:           My harddrive
            Content Hint:          Apple_HFS

…

For easier viewing use diskutil cs list | grep Conversion to only show the line with the progress.

i like to do it this way.

while true; do diskutil cs list | grep 'Conversion Progress' ; sleep 30; done

will print out an updated progress every 30 seconds so you can just leave it running and glance over and instantly see where the progress is up to without having to run the command again.