How do I access a locked hard drive with an incorrect password?

In general, the default password will be the login password of the Admin user who invoked encryption or was logged in when the drive was added.

it is also possible to have create a unique password for the drive.

There is no back door or work-around. Also, these passwords iare never saved "in the clear". All your Mac can tell you is, when a candidate password is entered and encrypted in the same way as the correct password is, "was that it?' Your Mac does not even knew the password length.

If the password cannot be remembered, those files are dead to you (and everyone else) and encryption has done the job you asked of it.

The password is essentially the encryption key. Without it, you will not be able to access the drive.