Disk enclosure

Lore & Background

The disk enclosure, also referred to as a caddy, is a sheath—typically plastic or metallic—within which a hard disk drive can be placed and connected with the same type of adapters as a conventional motherboard and power supply would use. The exterior of the caddy typically has two female sockets, used for data transfer and power. Variants of caddies exist: some larger caddies can support several devices at once and can feature either separate outputs to connect each device to a different computer, or a single output to connect both over the same data cable. Some caddies do not require an external power supply, instead obtaining power from the device to which they are connected; others have integrated fans to keep the drives cool. Caddies for all major standards exist, supporting for example ATA, SCSI, and SATA drives and USB, SCSI, and FireWire outputs.

Form factors for enclosures include multiple-drive RAID-enabled and iSCSI enclosures, as well as single-drive sizes such as 5.25-inch, 3.5-inch, 2.5-inch, and 1.8-inch. The 3.5-inch form factor was introduced with the Apple Macintosh series in 1984 and later adopted widely with the IBM PS/2 series in 1987; it is today used by most desktop hard drives. The 2.5-inch form factor is widely used in notebook computers and similar small-footprint devices, with radically lower power consumption that often enables enclosure vendors to power the devices directly from the host device's USB or other external bus. The 1.8-inch form factor is found in extremely compact devices such as certain portable media players and smaller notebooks, but is not standardized like the 2.5-inch cousins.

In enterprise storage, the term refers to a larger physical chassis used in network-attached storage (NAS), components of a storage area network (SAN), or directly attached to one or more servers over an external bus (DAS). These devices may include a backplane, temperature sensors, cooling systems, enclosure management devices, and redundant power supplies.

Reader's Guide

Disk enclosures have played a significant role in expanding storage capabilities across consumer and enterprise computing. Key benefits include adding additional storage space and media types to small form factor and laptop computers, as well as sealed embedded systems such as digital video recorders and video game consoles. They enable adding RAID capabilities to computers that lack RAID controllers or adequate space for additional drives, and allow adding more drives to any given server or workstation than their chassis can hold. Enclosures facilitate transferring data between non-networked computers (jokingly known as sneakernet) and provide an easily removable backup source with a separate power supply from the connected computer. Network-attached storage-capable enclosures allow sharing data over a network or providing a cheap off-site backup solution. Enclosures can prevent the heat from a disk drive from increasing the heat inside an operating computer case, offer a simple and cheap approach to hot swapping, and aid in recovering data from a damaged computer's hard drive—particularly when it does not share the same interface with the computer used to perform the recovery. They lower the cost of removable storage by reusing hardware designed for internal use, and in some instances provide a hardened chassis to prevent wear and tear. Consumer enclosures commonly use USB, FireWire, or Serial ATA connections, while enterprise enclosures may use SCSI, SAS, Fibre Channel, eSATAp, or eSATA interfaces. Network protocols such as iSCSI, NFS, or CIFS allow an external hard drive to send data over a network. The legacy of disk enclosures lies in their versatility: they have made external storage affordable, portable, and adaptable across diverse computing environments.