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The head arms are thin pieces of metal, usually triangular in shape onto which the head sliders (carrying the read/write heads) are mounted. In a way, the idea here is similar to how the arm of a phonograph is used to move the stylus from the outside of a record to the inside (although of course the similarity ends there). There is one arm per read/write head, and all of them are lined up and mounted to the head actuator to form a single unit. This means that when the actuator moves, all of the heads move together in a synchronized fashion. Heads cannot be individually sent to different track numbers.
The arms themselves are made of a lightweight, thin material, to allow them to be moved rapidly from the inner to outer parts of the drive. Newer designs have replaced solid arms with structural shapes in order to reduce weight and improve performance. This is the same technique used to reduce weight in the construction of airplane wings, for example. Newer drives achieve faster seek times in part by using faster and smarter actuators and lighter, more rigid head arms, allowing the time to switch between tracks to be reduced.
A recent trend in the hard disk industry has been the reduction in the number of platters in various drive families. Even some "flagship" drives in various families now only have three or even two platters, where four or five was commonplace a year or so ago. One reason for this trend is that having a large number of head arms makes it difficult to make the drive with high enough precision to permit very fast positioning (on random seeks). This is due to increased weight in the actuator assembly from the extra arms, and also problems aligning all the heads. So in essence, this is a tradeoff that some drive manufacturers are making to improve performance at the expense of capacity. With drive densities now at 20 GB per platter and bound to increase, this is an acceptable design decision for most buyers.
Next: Head Actuator