I don't see any way or reference on how to decrypt an encrypted drive.

>System Preferences>Security & Privacy>Filevault

The only way to do this is the Encryption has to fully complete first, only then can you decrypt the drive.

Turn off FileVault encryption on Mac - Apple Support

mistralski wrote:

These two drives have been encrypted for years, so your reply doesn't apply.

Were these drives encrypted with some application other than FileVault?

If so, you will need to address this issue with that application.

If not, then by virtue of turning FileVault off, the drives should automatically be decrypted by the OS without further intervention by you.

Have you been using them during these years?

These are external drives?

Right-click (or control-click) the drive image in the sidebar of a Finder window.

Do you now have an option to decrypt the drive?

Apparently, FileVault doesn't do external drives.

How to Decrypt External Drives on Mac - OSXDaily

FileVault status can be obtained via an Applications -> Utilities -> Terminal session

fdesetup status

    FileVault is On.

    Encryption in progress: Percent completed = 44.48

diskutil cs list | grep 'Conversion Progress'

    Conversion Progress:   44%

    Conversion Progress:   Optimizing 48%

FileVault disable in the middle of encrypting via an Applications -> Utilities -> Terminal session

sudo fdesetup disable

Password: <your admin password, then Return key; it will not echo, but it will be read>

It starts with you having a valid password, and the same software solution that encrypted the drive, so it will be compatible decrypting it with that password.

If drives could be decrypted without the password and app that would defeat the purpose of encryption. You should not be able to decrypt the drive without the correct password or assistance from the encryption app developer who may know of a bypass procedure.

mistralski wrote:

I finally managed to decrypt the drives. I found an obscure reference to booting with the "R" option - then the File Vault program brought up the decrypt option. However, I could not find a way to select a drive - it only decrypts the boot drive. Dreadful - took me days screwing around - something that should have been obvious!!!

You can see Volume UUID of all volumes if you use the terminal method, copy and paste:

diskutil info -all

https://osxdaily.com/2016/10/25/decrypt-external-drive-mac/

https://www.stellarinfo.com/blog/how-to-encrypt-decrypt-and-recover-mac-hard-drive/

Apparently I'm late to the party. 😄