I speculate that you have clicked options like Remember Me or Remember MY Password or other similar, freeing you from typing the password to decrypt the external hard drive. You can cancel off the Remember Me function on your device to let it ask for a password for the decryption. If this method makes no sense, set a password again for further security and encryption continuously.
If both two measures fail, you can back up the drive, format the external hard drive, transfer files back to it, and then enable the encryption to make it only accessible to people who have a correct passcode.