D-Link DNS-320L User Manual page 389

Section 5 - Knowledge Base
RAID 0 provides data striping, which spreads out blocks of data over all the
drives, but does not provide data redundancy.
Although performance is improved, the lack of fault tolerance means that if
one drive fails, all data in the array will be lost.
RAID 1 provides mirroring over multiple disks, with the same read/write
speed of a single disk. A RAID 1 array can only be as large as it's smallest
member disk.
Because the data is stored on multiple disks, RAID 1 provides fault tolerance
and protection, in addition to performance advantages.
D-Link ShareCenter DNS-320L User Manual 383