Abstract: A method of determining the write head capacity during self test of a hard disk drive is described. Information related to the dynamic coercivity of the disk service is obtained. In one embodiment, a DC erase condition is produced onto the disk surface. An AC signal is provided to a write head such that a region on the disk surface is put into a state of substantially no direction of magnetization. The AC signal to cause this state will be related to the coercive current. The coercive current can be used to estimate the required write current at low temperatures and thus determine whether to reject the hard disk drive.