WD says that the track width on their newest drives is about 50nm. The servo has to position the head with an accuracy considerably smaller than that, especiall

WD says that the track width on their newest drives is about 50nm. The servo has to position the head with an accuracy considerably smaller than that, especially for writes - for reads you can just try again if you're off center. (which is why some drives have different seek time numbers for read and write - they don't need as much "settling time" for reads) For comparison, green light has a wavelength of about 550nm.