D-Link DNS-320L User Manual page 391

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.
D-Link ShareCenter DNS-320L User Manual 385