1. A computer system comprising: a first host computer that executes an application program, a storage clustering control program and a copy control program; a first storage apparatus, which is coupled to the first host computer, and which provides a first volume; a second storage apparatus, which is coupled to the first host computer and the first storage apparatus, and which provides a second volume; and a third storage apparatus, which is coupled to the first storage apparatus and the second storage apparatus, and which provides a third volume, wherein the first storage apparatus, by executing a synchronous remote copy process for a first copy pair that has the first volume as a copy source and the second volume as a copy destination, sends, to the second storage apparatus, write data that the host computer has sent in accordance with the execution of the application program, wherein the first storage apparatus, by executing an asynchronous remote copy process for a second copy pair that has the first volume as a copy source and the third volume as a copy destination, sends, to the third storage apparatus, write data that the host computer has sent in accordance with the execution of the application program, wherein the first host computer, by executing the storage clustering control program, detects that the write of the write data has ended abnormally, and sends, to the second storage apparatus, a first instruction which causes the second storage apparatus to enable a write of the write data after detection to the second volume by changing a pair status of the first copy pair, and the first host computer, by executing the copy control program, detects that the abnormal end has been detected by the storage clustering control program subsequent to the first instruction having been sent, and sends, to the second storage apparatus, a difference resync instruction for commencing a copy process in accordance with a different asynchronous remote copy process for a third copy pair that has the second volume as a copy source and has the third volume as a copy destination.