Abstract: A hard disk drive prevents a data read error by optimizing a bias current supplied to an MR-element. Signal patterns of given amplitudes are written in a data area of the recording media, and sample-and-hold values of a read signal in the above signal patterns are read and stored while changing the bias current supplied to a selected MR-element. The stored sample-and-hold values are searched to select a sample-and-hold value equal to the amplitude of the signal patterns written in the data area, and the bias current supplied to the MR-element is determined as an optimal value to obtain the selected sample-and-hold value. The optimal value of the bias current may be written in a maintenance area of the recording media and is then read upon power up of the hard disk drive.