A spindle motor, which rotates a plurality of data storage disks in a hard disk drive, the spindle motor including: a shaft; an inner hub rotatably installed on an outer periphery of the shaft; and a hub assembly, which includes: an outer hub detachably coupled to an outer periphery of the inner hub, the outer hub shaped such that a plurality of disks and at least one spacer for maintaining an interval between adjacent disks of the plurality of disks can be secured to an outer periphery of the outer hub; and a fastening member coupled to a top portion of the outer hub and fixing the outer hub to the inner hub. The outer hub assembly to which the plurality of disks are secured can be mounted on an off-line servo-track writer, and can also be attached as is to the spindle motor after servo-track writing is completed.