Embodiments of the invention provide a small, thin, yet reliable magnetic disk drive. In one embodiment, a magnetic disk drive comprises: a motor hub including a disk support portion and a flange portion, the disk support portion supporting a magnetic disk thereon, the flange portion being located higher than the disk support portion and protruding radially outward with respect to a rotational center of the magnetic disk; a clamp having a ring shape and including an inclined undersurface inclined downward from the outer circumference of the ring shape toward the inner circumference of the ring shape, the ring shape encircling the flange portion, the clamp being engaged around the outer circumference of the flange portion; and an O-ring engaged with the inclined undersurface of the clamp so as to press the clamp against the flange portion or clamp the clamp, the O-ring being further engaged with the magnetic disk so as to press the magnetic disk against the disk support portion.