1. A computer system comprising: a first host computer which executes an application program, a storage clustering control program, and an asynchronous copy control program, which is a separate program from the storage clustering control program, the first host computer sending write data in accordance with the execution of the application program; a first storage apparatus which provides a first volume for storing the write data; sends the write data to a second storage apparatus by executing a synchronous remote copy process for a first copy pair that has the first volume as a copy source and a second volume as a copy destination; and creates first journal entries from the write data and sends the first journal entries to a third storage apparatus by executing an asynchronous remote copy process for a second copy pair that has the first volume as a copy source and a third volume as a copy destination, wherein the second storage apparatus is configured to provide the second volume, store the write data to the second volume, and create second journal entries from the write data sent from the first storage apparatus; and the third storage apparatus provides the third volume and stores the write data sent as the first journal entries to the third volume, wherein the first host computer, by executing the storage clustering control program, detects a failure of writing the write data to the first volume and sends a first instruction to the second storage apparatus, so that the second storage apparatus enables writing the write data to the second volume by changing a pair status of the first copy pair and continuing to create the second journal entries, and wherein the first host computer, by executing the asynchronous copy control program, asynchronously detects information from the asynchronous copy control program, which information indicates that the failure has occurred and that the first instruction has been sent; and sends a difference resync instruction to the second storage apparatus, so that the second storage apparatus starts a different asynchronous remote copy process for a third copy pair that has the second volume as a copy source and the third volume as a copy destination, by sending second journal entries created after receiving the first instruction and before receiving the difference resync instruction.