D-Link DNS-327L User Manual page 463

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-327L 458
D-Link ShareCenter User Manual