El Capitan - Filevault encrypting external hard drive?

[fessen]

I've been waiting now probably 18 hours or more for a backup copy of an El Capitan MBP drive to be encrypted, but I still can't tell whether the encryption process has completed.
How to tell? And how long should the process take?

background:
I cloned a filevault-encrypted MBP hard drive to an external hard drive using Carbon Copy Cloner.
I confirmed that the cloned copy was bootable.

Then, while booted into the external copy, I started filevault encrypting the external hard drive.

I had read the following in the Carbon Copy Cloner documentation:

You do not have to wait for the conversion process to complete before using the backup disk. Additionally, you do not have to remain booted from the backup disk for the conversion process to complete. You can simply enable FileVault encryption, then immediately reboot from your primary startup disk and the conversion process will carry on in the background. Encryption will continue as long as the backup disk is attached. OS X doesn't offer a convenient method to see conversion progress, but you can type diskutil cs list in the Terminal application to see conversion progress.

so a few moments after starting the encryption process, I rebooted back into the MBP El Capitan installation and left the computer and external drive on overnight.

How long should encrypting an external hard drive clone take?
The total drive capacity is 4TB, but the partition set aside for the El Cap clone is only 1.4TB.
(Incidentally, the El Capitan files take up only about 0.7TB of that.)

If I try to boot into the external hard drive copy, I get an open circle with a diagonal (forbidden) slash down the middle.

When I boot into the main MBP hard drive, and then put in my password to unlock the external hard drive, the ElCap partition on the external hard drive doesn't show up in Finder, although the unfilled partition on the external drive does show up. And

diskutil cs list

shows

Name:         ElCapCopy
    Status:       Online
    Size:         1399349993472 B (1.4 TB)
    Free Space:   18898944 B (18.9 MB)
    |
    +-< Physical Volume 2.................................
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk2s2
    |   Status:   Online
    |   Size:     1399349993472 B (1.4 TB)
    |
    +-> Logical Volume Family 9....................................
        ----------------------------------------------------------
        Encryption Type:         AES-XTS
        Encryption Status:       Unlocked
        Conversion Status:       Pending
        High Level Queries:      Not Fully Secure
        |                        Passphrase Required
        |                        Accepts New Users
        |                        Has Visible Users
        |                        Has Volume Key
        |
        +-> Logical Volume 1.......................................
            ---------------------------------------------------
            Disk:                  disk3
            Status:                Online
            Size (Total):          1398978772992 B (1.4 TB)
            Conversion Progress:   Paused
            Revertible:            Yes (no decryption required)
            LV Name:               ElCapCopy
            Content Hint:          Apple_HFS

In Disk Utility, the external hard drive El Capitan volume is greyed out, but the info display shows:

Volume name - ElCapCopy
Volume type - Logical Volume
File system - OS X Extended
Connection - USB
Writable - No
Owners enabled - No
Is encrypted - Yes
Can be verified - Yes
Can be repaired - Yes
Bootable - No
Journaled - No