A hard disk drive (HDD) is a device for writing and reading digital data.  Data is magnetically read from and written to rotating disks by read and write heads that hover close to the surface of the disk.  An arm moves the heads across the disks as they spin, allowing each head to access various locations on the surface of the disk. 
A servo system is used to control the movement of the arm and the rotation of the disk.  Servo code is written to certain portions of the HDD and is used by the servo system to control the position of the heads when they read and write.  Because of this, the accuracy and precision of where the servo code is written is important.  Improved techniques for servo writing would be desirable.