NAS is network-attached storage, such as the DS1520+, whereas DAS stands for directly-attached (via USB, Thunderbolt, eSATA, etc.) storage. A regular external hard drive or the LaCie 5g Thunderbolt are DAS devices. As a rule of thumb, directly-attached storage is usually faster because USB 3, Thunderbolt or eSATA offer higher speeds than standard (1Gbe) ethernet links.