<para> A reliability model for the hard disk drive (HDD) is developed, focusing on head–disk separation as the primary independent variable. The model is structured to incorporate the theoretical effects of environmental factors, plus empirical dependence on the product operating mode. An experimental method based on magnetic spacing loss theory is used to characterize the head–media separation as a function of temperature, altitude, humidity, and HDD operating mode. A statistical model based on these empirical data is developed to predict HDD reliability for various operating conditions. The predictions of the model are verified experimentally through comparison with HDD product reliability test data. </para>