A method or apparatus can reduce or prevent unauthorized data access by securing data stored on a hard disk of a computer system. The method can include requesting a user to enter a password if the computer system is powered on under the condition that a password for security of the hard disk of the computer system is previously stored and data previously written on the hard disk is converted into a specific format for security and stored on the hard disk of the computer system, determining whether the password entered by the user corresponds to the previously stored password, storing the determined result, and selectively reading and outputting the format-converted data stored on the hard disk upon the user requests according to the determined result. The requested data can be output to a monitor. User write operation requests can operate according to the determined result.