The mechanical arm that moves the read/write head across the surface of a disk similar to a tone arm on a phonograph. The access arm is directed by instructions in the operating system to move the read/write head to a specific track on the disk. The rotation of the disk positions the read/write head over the required sector. The access arm is also called an "actuator arm," because the actuator is the motor and mechanism that moves the arm. See read/write head.