A data replication system comprises a NAS controller and a storage system. In a first embodiment, after a NAS system takes a snapshot of a volume, the NAS system changes the referral information within the volume. Also, as an additional way to ensure the consistency after taking a snapshot, the NAS System returns the changed referral information based on a referral management table when clients access a referral within the snapshot volume so that clients can access an appropriate volume. In a second embodiment, after a volume on a NAS system is replicated by remote copy to a remotely deployed NAS system, the remotely deployed NAS system returns the changed referral information based on a referral management table when clients access a referral within the replicated volume so that clients can access an appropriate volume.