D-Link DNS-320L User Manual page 367

Section 5 - Knowledge Base
RAID 1 - allows you to assign two disks as a mirrored set. Once you create
the mirrored set, you will see it as a single disk drive. But when you write
data to the mirrored set, it will duplicate the data across all members of
the set. This ensures that your data is protected against loss if any hard
drive in the RAID 1 set fails. In fact, as long as any single member of the set
remains functional, you will continue to operate normally, with complete
access to your data.
DNS-320L 362
D-Link ShareCenter™ User Manual