During the process of seek from one track to another, the error between the position of the head and the destination track gradually becomes smaller. However, it is practically impossible to bring the error to zero and maintain it there from that time onward. Even though the head positioning servomechanism tries to make the head follow the center of a track while reading or writing data, it is practically not possible to make the error zero.
So the end of a seek process does not imply zero position error. In fact, the seek is assumed to come to an end if the position error remains less than some pre-specified limit for few consecutive samples. The limit is typically 15% of the track pitch before a reading operation is allowed, and 10% of track pitch for a seek prior to writing data. This is a major difference from the typical definition of settling time in control system step responses. In HDD servo mechanism, the error must be less than 10%(writing) or 15%(reading) of a single track irrespective of the number of tracks traversed by the seek operation. Let us consider a seek command asking for movement of the head from track N to track N + 100, which is equivalent to a step response with yref = 100. According to the definition of 5% settling time in linear control system, the settling time is equal to the time it takes to bring the position error within ±5 tracks. And it is equal to ±10 tracks for a seek command of 200 tracks. However, in HDD servomechanism, the limit of position error is 10% (writing) or 15% (reading) of one track for all seek lengths.