A computer system for preventing data loss that includes: a first storage system having a data storage extent for storing data sent from a host computer; a second storage system having at least one actual replicated data storage extent associated with the data storage extent; an archive appliance having a storage medium associated with the replicated data storage extent in the second storage system; and a data copy unit for controlling copy processing for reading data from the data storage extent in the first storage system and writing the read data to a first actual replicated data storage extent in the second storage system in accordance with specific configuration information. The system also has a connection switching unit for changing the specific configuration information so that a second actual replicated data storage extent, instead of the first actual replicated data storage extent, is associated with the data storage extent.