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 .