External storage device capable of selectively storing data in a semiconductor memory or a hard disk

An external storage device, comprising:

• a storage medium;
• a second storage medium having an access speed lower than said first storage medium, wherein said second storage medium includes a backup storage area capable of storing data from said first storage medium;
• judgment means for determining, in response to a write access to said external storage device, whether or not data from said write access can be stored in said first storage medium;
• access processing means for compressing and storing said data from said write access into said first storage medium when said data can be stored in said first storage medium, and for storing said data into said second storage medium when said data cannot be stored in said first storage medium;
• and
• transfer means for transferring data from said first storage medium to said backup storage area in said second storage medium in response to a power outage or abnormality occurred in said extornal storage device and said second storage medium operates normally, and for transferring said data from said backup storage area back to said first storage medium in response to a power restoration on said external storage device after said power outage or abnormality, wherein power consumption of said second storage medium is suppressed when in response to said power outage or abnormality and said second storage medium does not normally operate.