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The part of a hard disk to which the read/write arms attaches. All the heads are attached to a single head actuator, also called an actuator arm, that moves the heads around the platters.  Older hard drives used a stepper motor actuator, which moved the heads based on a motor reacting to stepper pulses. Modern hard drives use a voice coil actuator, which controls the movement of a coil toward or away from a permanent magnet based on the amount of current flowing through it. This guidance system is called a servo.

See also Webopedia's "Did You Know... What's Inside a Hard Drive?"

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