Is it OK for Time Machine to take a long time to decrypt a freshly formatted encrypted disk?

Question

I'm new to Time Machine. I just this week got an external hard disk and am starting to use it. My Time Machine backups were spending a significant amount of time in the encryption phase. Upon reading this: https://www.howtogeek.com/305540/how-to-encrypt-your-macs-time-machine-backup/

I thought a better option would be to format the disk as Mac OS Extended (Journaled, Encrypted) instead of the corresponding unencrypted format. If I understand things right I could then do decrypted backups via Time Machine instead of encrypted backups, leaving the encryption up to the OS's writing to the disk via the encrypted file system format. It seems, judging from the question and answers here, that what I'm doing should be reasonable:

Difference between enabling Time Machine's "Encrypt Backups" option, and encrypting from Disk Utility?

So I erased my external drive which only had a few days backups anyway and reformatted it as Mac OS Extended (Journaled, Encrypted). But when I added the disk via Time Machine, selected to not encrypt it, and entered my encryption password, I get a message saying this:

This Core Storage logical volume is already decrypting.

Wait, what? It's decrypting? Why does it need to decrypt anything? The disk is formatted, empty, and ready to fill with data, right? I suppose it doesn't know how full or empty the disk is because the entire disk is encrypted? It has been running, decrypting, for quite a few hours and diskutil cs list shows it as only 12% done:

$ diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
    =========================================================
    Name:         <name>
    Status:       Online
    Size:         2000021315584 B (2.0 TB)
    Free Space:   9392128 B (9.4 MB)
    |
    +-< Physical Volume XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk2s2
    |   Status:   Online
    |   Size:     2000021315584 B (2.0 TB)
    |
    +-> Logical Volume Family XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
        ----------------------------------------------------------
        Encryption Type:         AES-XTS
        Encryption Status:       Unlocked
        Conversion Status:       Converting (backward)
        Reversion State:         Decrypting
        High Level Queries:      Not Fully Secure
        |                        Has Visible Users
        |                        Has Volume Key
        |
        +-> Logical Volume XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
            ---------------------------------------------------
            Disk:                  disk3
            Status:                Online
            Size (Total):          1999659597824 B (2.0 TB)
            Conversion Progress:   12%
            Revertible:            No
            LV Name:               WDExternalDrive
            Volume Name:           WDExternalDrive
            Content Hint:          Apple_HFS

Note, in particular:

Reversion State:         Decrypting

and:

Conversion Progress:   12%

Can someone explain to me what is going on? I'm concerned I'm doing something wrong. When it says that it is decrypting, is it decrypting the freshly encrypted drive so that my snapshots will be stored plaintext on the drive? That's clearly not what I want.

---

Update

After waiting all weekend, the status now shows that the disk conversion process is completed:

$ diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
    =========================================================
    Name:         WDExternalDrive
    Status:       Online
    Size:         2000021315584 B (2.0 TB)
    Free Space:   9392128 B (9.4 MB)
    |
    +-< Physical Volume XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk2s2
    |   Status:   Online
    |   Size:     2000021315584 B (2.0 TB)
    |
    +-> Logical Volume Family XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
        ----------------------------------------------------------
        Encryption Type:         None
        |
        +-> Logical Volume XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX
            ---------------------------------------------------
            Disk:                  disk3
            Status:                Online
            Size (Total):          1999659597824 B (2.0 TB)
            Conversion Progress:   Complete
            Revertible:            No
            LV Name:               WDExternalDrive
            Volume Name:           WDExternalDrive
            Content Hint:          Apple_HFS

But now Time Machine won't let me use the disk because the volume is not encrypted.

Indeed, Disk Utility shows that the format is no longer encrypted:

So it looks like I did something wrong, which was my concern that led me to make the post in the first place. Oh well, it was just machine time. Is what I'm doing misguided? Did I miss a step somewhere or take a wrong turn?

My question still remains: can someone tell me what is wrong with my process? I want to use the filesystem encyrypted format to backup my machine without Time Machine needing to encrypt the disk as a post process operation. Is this possible?

Answer 1

I’ve had the same question, and after a bit of research (and based on my own experience) I’ve come to some conclusions:

1. Encryption made by TimeMachine and Encryption made by disk utility is the SAME thing and it is done only one time.

2. Based on your question I think that this part of TM is poorly implemented (and NOT explained by apple), in fact when you, after encrypting the backup, selected the option to NOT encrypt it in TM the system asked you for the encryption password and done a (I guess) total decryption of all the free memory on the disk, instead of simply reformat that (and I think that TM does that to be “transparent” in that operation without having to ask to reformat the disk in an encrypted format).

THEN TO ANSWER YOUR QUESTION

“ My question still remains: Can someone tell me what is wrong with my process? I want to use the filesystem encrypted format to backup my machine without Time Machine needing to encrypt the disk as a post-process operation. Is this possible?”

YES, this is possible, given what I said in “1.”, the best method that you can use (and that I use too) to achieve that is:

• Format the disk in “Mac OS Extended (Journaled) Encrypted”

• set the disk as the backup disk for TM, checking the option to set the backup as ENCRYPTED

Let it do the backup (which will not do additional encrypting processes etc.) and then you’re done!

Answer 2

In your case for 2 TB drive it can take up to few days.

To keep it working overnight go to System Preferences > Energy Saver and leave the disk running.

Of course leave it plugged in.

Just to make sure it stays awake, you can use the caffeinate in Terminal.

You can check the status from the terminal with diskutil cs info /Volumes/<your drive name> and look for Conversion State and LV Conversion Progress