1. A memory device, comprising: a first memory having a first storage capacity; a second memory having a second storage capacity greater than the first storage capacity; and a controller to provide an external host with an address space corresponding to a third storage capacity, the third storage capacity being less than a sum of the first storage capacity and second storage capacity, wherein the controller, where data requested from the external host is stored in the first memory, transmits the requested data to the external host from the first memory, and where the requested data is not stored in the first memory, transmits the requested data to the external host from the second memory.