Abstract: An access mechanism for a vertical stack of magnetic disks including a mechanism carriage moveable back and forth horizontally by a prime mover, an access arm having a magnetic head adjacent its end and carried by a carriage moveable along slide shafts mounted in the mechanism carriage at an angle with respect to horizontal and a stop against which the access arm carriage moves whereby the access arm moves vertically along the periphery of the disk stack as the movement of the mechanism carriage progresses after contact of the access arm carriage with the stop. A pawl carried by the mechanism carriage coacts with ratchet teeth carried by the access arm carriage for holding the access arm and magnetic head at selected vertical positions so that the access arm and head may be moved between a selected pair of disks in the stack by movement of the mechanism carriage in the opposite direction.