A portable data storage device is disclosed which includes an Interface for enabling the portable data storage device to be used for data transfer with a host Computer, and an Interface controller for controlling the interface. There is also a master control unit for controlling the writing of data to and reading data from a non-volatile memory. The non-volatile memory includes at least one single layer cell flash memory and at least one multiple layer cell flash memory. Upon receiving a write instruction, the master control unit determines which of the memories data contained in the instruction should be written to, and writes the data as appropriate similarly, upon receiving a read instruction, the master control unit reads the data from the appropriate one of the memories and transmits the data out of the device.