Complex sentence: In Maximum security mode, the disk cannot be unlocked without the User password--the only way to get the disk back to a usable state is to issue the SECURITY ERASE PREPARE command, immediately followed by SECURITY ERASE UNIT.
Split into simpler sentences: In Maximum security mode, the disk can be unlocked only with the User password. If the User password is not available, the only reamaining way to get at least the bare hardware back to a usable state is to issue the SECURITY ERASE PREPARE command, immediately followed by SECURITY ERASE UNIT.