Time Machine fails since enabling FileVault on iMac with Fusion Drive I have an iMac with a Fusion Drive, and Time Machine configured to back up to an external USB disk.
Everything used to work well, but then I enabled FileVault for the internal Fusion Drive. Since then, Time Machine has not been able to back up:

Clicking the red info button shows:

I left it for multiple days, rebooted the computer, reset NVRAM, did First Aid using Disk Utility for both the external disk and the filesystem in it, all to no avail. This is on macOS Big Sur 11.3.1 (20E241).
diskutil cs list shows that the encryption is complete.
kartick@iMac ~ % diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group B2E1C2E9-637C-4BAB-897B-ABA771347B48
    =========================================================
    Name:         Time Machine Backup
    Status:       Online
    Size:         5000637104128 B (5.0 TB)
    Free Space:   0 B (0 B)
    |
    +-&lt; Physical Volume 91CE72E6-ACB1-4B90-8DA2-FFE4365D4A4F
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk3s2
    |   Status:   Online
    |   Size:     5000637104128 B (5.0 TB)
    |
    +-&gt; Logical Volume Family 957C53B4-DB6F-4E1F-B743-286CAA1E75A6
        ----------------------------------------------------------
        Encryption Type:         AES-XTS
        Encryption Status:       Unlocked
        Conversion Status:       Complete
        High Level Queries:      Fully Secure
        |                        Passphrase Required
        |                        Accepts New Users
        |                        Has Visible Users
        |                        Has Volume Key
        |
        +-&gt; Logical Volume 0A5A21A4-D46E-4FE8-9663-3AA7EE5EA68E
            ---------------------------------------------------
            Disk:                  disk4
            Status:                Online
            Size (Total):          5000284778496 B (5.0 TB)
            Revertible:            No
            LV Name:               Time Machine Backup
            Volume Name:           Time Machine Backup
            Content Hint:          Apple_HFS

How do I get it to work?
The problem isn't the TM disk encryption, but the system disk Data &quot;Macintosh HD - Data&quot;.
You have to wait that the filevault encryption be full.
If your Mac is a laptop model, keep in mind that the encryption requires an electrical connection.
To verify the encryption 2 commands possible :
diskutil ap list or
fdesetup status