Direct-attached storage (DAS) is digital storage directly attached to the computer accessing it, as opposed to storage accessed over a computer network. Examples of DAS include hard drives, optical disc drives, and storage on external drives. The name "DAS" is a retronym to contrast with storage area network (SAN) and network-attached storage (NAS).
A typical DAS system is made of a data storage device (for example enclosures holding a number of hard disk drives) connected directly to a computer through a host bus adapter (HBA). Between those two points there is no network device (like hub, switch, or router), and this is the main characteristic of DAS.
Storage features common to SAN, DAS and NAS
Most functions found in modern storage do not depend on whether the storage is attached directly to servers (DAS), or via a network (SAN and NAS).
Advantages and disadvantages
The key difference between DAS and NAS (network-attached storage) is that DAS storage is only directly accessible from the host to which the DAS is attached. A DAS does not incorporate any network hardware and related operating environment to provide a facility to share storage resources independently. The storage presented by a DAS to a connected host can of course be shared by that host. A SAN (storage area network) has more in common with a DAS than a NAS with the key difference being that DAS is a 1:1 relationship between storage and host whereas SAN is many to many.