a magnetic disk apparatus includes a first disk drive unit capable of accessing a first - size disk contained in a first disk cartridge , and a second disk drive unit capable of accessing a smaller , second - size disk contained in a second disk cartridge , the second disk drive unit being attached to the first disk drive unit to form a single unit . the second disk drive unit comprises : a chassis having a size substantially equal to a size of the first disk drive unit ; a disk motor which rotates the second - size disk of the second disk cartridge on the disk motor when the second - size disk is accessed ; and a motor base attached to a bottom of the disk motor , the motor base including a control circuit to control the rotation of the disk motor . an entire top area of the first disk drive unit is covered with the chassis of the second disk drive unit , and an entire height of the second disk drive unit is substantially equal to an entire height of a conventional disk drive unit added to a thickness of a top plate .