Can you be more specific about what you mean by hard disk password? Are we talking about a hardware encryption device, software based hard drive encryption with boot time protection, or something else?
Are you talking about something specific or just asking in general? If its specific you might want to state what Operating System is being used, what does the password prompt look like, are there any brand names or product names shown at or before the prompt. If general you might want to provide details about where you read about it and what you know about it and how it works.
As it is your question is far too general and vague to elicit any useful answers.