1. A method of data transfer to and from a storage device, comprising: in a storage device having a memory, a controller, a host interface and a storage device interface, performing by the controller, when the host interface is connected to a host device and the storage device interface is coupled to an add-on memory: receiving a data transfer command from the host interface, the data transfer command specifying a memory address, wherein the memory has an address space; determining if the specified memory address is beyond the address space of the memory; and if the specified memory address is beyond the address space of the memory, routing the data transfer command to the storage device interface for access to the add-on memory, otherwise processing the data transfer command for access to the memory, wherein the address space of the memory and an address space of the add-on memory are addressed as one integral address space transparently to the host device.