A dynamic self-adjusting memory storage device and method of operating. The device includes a plurality of adjustable-size phase change memory (PCM) storage sub-modules connected to and communicating over a bus with a control device. One of the plurality of adjustable-size memory storage sub-modules is in a stand-by mode of operation. The control device implements steps to: determine, based on a switching criteria, when the memory storage device needs to be switched to a different operation mode; select one or more adjustable-sized memory storage sub-modules for switching to said different operation mode; copy stored data from a selected actively operating adjustable-size memory storage sub-module to said adjustable-size memory storage sub-module in said stand-by mode; and change the capacity of the selected actively operating adjustable-size memory storage sub-module after the copying. The dynamic self-adjusting memory capacity method is performed without powering down the memory storage device or paying any timing penalty.