Patent ID: 6845435

Claim:
A data storage system comprising: a first data storage apparatus connected to a host computer; and a second data storage apparatus configured to store data sent from said first data storage apparatus with an asynchronous copy mode or a synchronous copy mode; said first data storage apparatus having: a first data medium to store data sent from said host computer; a first cache memory; and a first controller configured to move data into and out of said first data medium and said first cache memory; said second data storage apparatus having: a second data medium to store data sent from said first data storage apparatus; a second cache memory; and a second controller configured to move data into and out of said second data medium and said second cache memory; wherein said asynchronous copy mode is used between said first data storage apparatus and said second data storage apparatus while said first controller does not detect probability of data loss in said first data storage apparatus, and wherein said copy mode changes from said asynchronous copy mode to said synchronous copy mode when said first controller detects said probability of data loss in said first data storage apparatus.