Turn off encryption on filevault2 second HDD
What methods have you used when trying to disabling the encryption on your encrypted non-boot drive? Here's some ways I know about:

Using Disk Utility


Boot your Mac and hold down ⌘-R (Command –R) to boot from the Mac’s Recovery HD partition.
Open Disk Utility.
Select your locked hard drive.
Under the File menu, select Turn Off Encryption…
When prompted for a password, you can enter your password for the drive. Once provided, decryption of the encrypted volume should begin.


The drive should begin decrypting.

From the command line


Boot your Mac and hold down ⌘-R (Command –R) to boot from the Mac’s Recovery HD partition.
Open Terminal.
Run the following command to get the UUID of the encrypted drive: diskutil corestorage list
Once you have the UUID, you can unlock the drive using the following command: diskutil corestorage revert UUID -stdinpassphrase
You’ll be prompted for the password/passphrase of an account that’s authorized to unlock the encryption. Once provided, decryption of the encrypted volume should begin.


The drive should begin decrypting.

For more information, see here:

Unlock or decrypt your FileVault 2-encrypted boot drive from the command line

Using Disk Utility to unlock or decrypt your FileVault 2-encrypted boot drive