1961: Separate Arms For Each Platter, With Heads Moving On Air
While the IBM 350 Disk File and several successors came with a single actuator arm and magnetic head that moved up and down the stack of platters to do the data reads and writes, the IBM 1301 Disk Storage Unit featured a separate arm and head for each platter. All the arms moved in and out together. This eliminated the need to remove the arm/head from one platter and move it to another.
The IBM 1301 was also the first hard drive whose heads rode a thin layer of air caused by the spin of the platters. This kept the heads closer to the platters to increase storage density; it could store 28 million characters per module on 20 platters.