Embodiments of the invention compensate for repeatable run-out errors
without causing servo system instability. In one embodiment, HDD has a
peak filter on a feedback route of a servo system. The peak filter is
designed so that a rotating frequency of a magnetic disk, the
high-frequency components contained in the rotating frequency and a peak
match the rotating frequency and the high-frequency components. Insertion
of a required peak filter into the servo system allows compensation for a
repeatable run-out (RRO) error due to an event such as a deviation from
the roundness of a track. Also, since a Nyquist diagram of the system
satisfies required characteristics, a repeatable run-out error may be
compensated for without causing instability of the servo system due to
use of the peak filter.