How to turn off encryption on previously encrypted HFS+ drive?

Question

The drive mounts as PlexMedia_1 at 18TB. I have another drive that mounts as PlexMedia_2 that is 6TB. I am unclear why the Name field is populated with PlexMedia_2 and not sure how to correct that.

/dev/disk9 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *6.0 TB     disk9
   1:                        EFI EFI                     209.7 MB   disk9s1
   2:                  Apple_HFS PlexMedia_2             6.0 TB     disk9s2

/dev/disk11 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *18.0 TB    disk11
   1:                        EFI EFI                     209.7 MB   disk11s1
   2:          Apple_CoreStorage PlexMedia_2             18.0 TB    disk11s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk11s3

/dev/disk15 (external, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS PlexMedia_1             +18.0 TB    disk15
                                 Logical Volume on disk11s2
                                 3AE5D9ED-9930-4207-82B1-2D912E3B94E6
                                 Unlocked Encrypted

Anyway, I want to remove encryption from this drive. I found this command but it fails

john@Mac-mini ~ % diskutil corestorage revert 4AD9490F-9214-4443-BCA5-B1899D46E838 -stdinpassphrase
diskutil: did not recognize coreStorage verb "revert"; type "diskutil coreStorage" for a list

The man page says that coreStorage commands are deprecated. To use apfs commands but this is not an APFS formatted HDD.

It's not APFS so diskutil apfs commands failed.

john@Mac-mini ~ % diskutil apfs decryptVolume /Volumes/PlexMedia_1  -stdinpassphrase
disk15 is not an APFS Volume
john@Mac-mini ~ % diskutil apfs decryptVolume /Volumes/PlexMedia_2  -stdinpassphrase
disk9s2 is not an APFS Volume

+-- Logical Volume Group 4AD9490F-9214-4443-BCA5-B1899D46E838
    =========================================================
    Name:         PlexMedia_2
    Status:       Online
    Size:         17999863963648 B (18.0 TB)
    Free Space:   0 B (0 B)
    |
    +-< Physical Volume 620D5ED2-290F-44E6-ACE9-91EEF790F0CF
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk11s2
    |   Status:   Online
    |   Size:     17999863963648 B (18.0 TB)
    |
    +-> Logical Volume Family 650EA5C9-56B9-4220-A956-19338567C05E
        ----------------------------------------------------------
        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
        |
        +-> Logical Volume 3AE5D9ED-9930-4207-82B1-2D912E3B94E6
            ---------------------------------------------------
            Disk:                  disk15
            Status:                Online
            Size (Total):          17999511638016 B (18.0 TB)
            Revertible:            No
            LV Name:               PlexMedia_1
            Volume Name:           PlexMedia_1
            Content Hint:          Apple_HFS

Answer 1

The fact that the following identifiers are associated with the following names is not a problem. I do not believe there is a way to change the name associated with the identifier disk11s2.

Identifier	Name	Description
disk9s2	PlexMedia_2	Journaled† HFS+ volume
disk11s2	PlexMedia_2	CoreStorage logical volume group
disk15	PlexMedia_1	Journaled HFS+ volume

^†Being journaled is an assumption I made. Actually, there is nothing in your question to confirm this.

---

Commands to Turn Off Encryption Using Your Version of macOS

This method will decrypt the volume. However, the result will be an APFS volume instead of the desired HFS+ volume.

First, enter the command below to convert to APFS.  

diskutil apfs convert disk15

Next, enter the command below to remove encryption. Note, you should verify the APFS volume identifier is disk15s1.

diskutil apfs decryptVolume disk15s1

Finally, use the command below to see progress.

diskutil apfs list

---

Command to Turn Off Encryption Using an Older Version of OS X/macOS

As you have already discovered, your version of macOS does not support diskutil cs revert. Note that the revert sub-verb does not work when the CoreStorage logical volume is not revertible (as shown in the output posted in your question). Therefore, even if you had access to a version of OS X/macOS that supported diskutil cs revert, the 18 TB drive could not be changed to be setup like the 6 TB drive without first erasing the data on the drive. In other words, you cannot remove the use of CoreStorage while preserving data stored on the volume.

However, if you just what to remove the encryption while keeping the use of CoreStorage, then you could use the command below from a version of OS X/macOS that supports the sub-verb decryptVolume. I believe the sub-verbs revert and decryptVolume were removed from the verb cs with the release of macOS Big Sur.

diskutil cs decryptVolume disk15

Note: You should use the command diskutil cs list to confirm the conversion progress is complete.

---

Using Finder Application to Decrypt

User John posted an answer showing the Finder application can be used to decrypt the volume. This method will remove the encryption while keeping the use of CoreStorage. I was curious when this method was added to OS X/macOS. Wikipedia states CoreStorage was introduced first with Lion (OS X 10.7). However, when running the current version of Lion (OS X 10.7.5) in VirtualBox, I found the Finder could not be used to decrypt a CoreStorage encrypted USB flash drive. When running the current version of Mountain Lion (OS X 10.8.5), I was able to use the Finder application to decrypt the flash drive, as shown below.

The process of decrypting can take a while. During the process, the Finder application will show the following. Basically, “Encrypting” is grayed out.

Below is what the Finder application will show when finished. Basically, “Encrypting” has been replaced by “Encrypt”, which is not grayed out.

Answer 2

We can decrypt a volume using... Finder ?!

Right click on the HDD in question and choose "Decrypt". Really the last place I would look for decrypting a HDD.