A storage device with a memory, a controller, a storage device interface and a host interface, and a method of data transfer to and from a storage device are provided. The storage device is operative to connect with a host device and the storage device interface is operative to couple with an add-on memory. When the host interface is connected to a host device, the controller is configured to receive a data transfer command specifying a memory address from the host interface; and if the memory address is beyond the address space of the memory, to route the data transfer command to the storage device interface for access to the add-on memory. 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.