A disk enclosure is essentially a specialized chassis designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the data sent across their native data bus into a format usable by an external connection on the computers to which it is connected. In some cases, the conversion is as trivial as carrying a signal between different connector types. In others, it is so complicated as to require a separate embedded system to retransmit data over connector and signal of a different standard. Factory-assembled external hard disk drives, external DVD-ROM drives, and others are all built around disk enclosures. Bulkier models built around 2.5" and 3.5" hard drives and full-height 5.25" DVD-ROM drives use enclosures that are often nearly identical to OEM enclosures.
Key benefits to using external disk enclosures 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.
- Adding more drives to any given server or workstation than their chassis can hold.
- Transferring data between non-networked computers, jokingly known as sneakernet.
- Adding a backup source with a separate power supply from the connected computer.
- Sharing the data on a drive in a network-aware enclosure.
- Preventing the heat from a disk drive from increasing the heat inside an operating computer case.
- Simple and cheap approach to hot swapping.
- Recovering the data from a broken or damaged computer.
- Lower the cost of removable storage by reusing hardware designed for internal use.
- More memory than CDs, DVDs, and Flash Drives.
- Better protection of files from damage, age, weathering, and corruption, than CDs, DVDs, and Flash Drives.
In the consumer market, commonly used configurations of drive enclosures utilize magnetic hard drives
or optical disc drives
inside of USB
, or Serial ATA
enclosures. External 3.5" floppy drive
are also fairly common, following a trend to not integrate floppy drives into compact and laptop
computers, started by Apple Computer
with their iMac
. Pre-built external drives are available through all major manufacturers of hard drives, as well as several third-parties.
- "5.25 inch" drive: (5.75 in x 1.63 in x 8 in = 146.1 mm x 41.4 mm x 203 mm)
Most desktop models of drives for optical 120-mm disks (DVD-ROM or CD-ROM drives, CD or DVD burners), are designed to be mounted into a so-called "5.25-inch slot", which obtained its nickname because this slot size was initially used by drives for 5.25-inch diameter floppy disks in the IBM PC AT. (The original "5.25-inch slot" in the IBM PC was with 3.25 in (82.6 mm) twice as high as the one commonly used today.)
- "3.5 inch" drive: (1 in x 4 in x 5.75 in = 25.4 mm x 101.6 mm x 146.05 mm)
This smaller, 4-inch wide disk-drive form factor was introduced with the IBM PS/2 series in 1987, which included drives of this size for 90-mm ("3.5-inch") floppy disks. This form factor is today used by most desktop hard drives. They usually have 10 mounting holes with American 6-32 UNC 2B threads: three on each side and four on the bottom.
- "2.5 inch" drive: (0.374 in x 2.75 in x 3.945 in = 9.5 mm x 69.85 mm x 100.2 mm)
This even smaller, 2.75-inch wide form factor is widely used today for hard disks in many mobile devices (laptops, music players, etc.)
- "1.8 inch" drive: These are used in Apple's iPod Classic. There are several USB enclosures with such hard drives (eg. by LaCie).
A range of other form factors has emerged for mobile devices. While laptop hard drives are today generally of the 9.5 mm high variant of the "2.5 inch" drive form factor, older laptops and notebooks had hard drives that varied in height, which can make it difficult to find a well-fitting chassis. Laptop optical drives require "slim" 5.25" enclosures, since they have approximately half the thickness of their desktop counterparts, and most models use a special 50-pin connector that differs from the 40-pin connectors used on desktop ATA drives.
While they are less common now than they once were, it is also possible to purchase a drive chassis and mount that will convert a 3.5" hard drive into a removable hard disk that can be plugged into and removed from a mounting bracket permanently installed in a desktop PC case. The mounting bracket carries the data bus and power connections over a proprietary connector, and converts back into the drive's native data bus format and power connections inside the drive's chassis.
In enterprise storage
the term disk enclosure
may refer to:
Native drive protocols
, Fibre Channel
, and eSATA
protocols can be used to directly connect the external hard drive to an internal host adapter
, without the need for any intervening controller. These native external drive protocols
are extremely similar to the internal protocols, but expanded to carry power. This is the case with eSATA and the SCSI Single Connector Attachment
standard. A host adapter
with external port may be necessary to connect a drive, if a computer lacks an available external port.
Direct attach serial protocols
connections are typically used to attach consumer class external hard drives to a computer. Unlike SCSI, eSATA, or SAS these require circuitry to convert the hard disk's native signal to the appropriate protocol. Parallel ATA
and internal Serial ATA hard disks are frequently connected to such chassis because nearly all computers on the market today have USB or FireWire ports, and these chassis are inexpensive and easy to find.
, or Windows File Sharing
are all commonly used protocols that are used to allow an external hard drive to use a network to send data to a computer system. This type of external hard drive is also known as Network-attached storage
or NAS. Often, such drives are embedded computers
running operating systems such as Linux
that use their NFS
daemons and SAMBA
to provide a networked file system. A newer technology, Network-Attached Storage
(NAS), has been applied to some disk enclosures, which allows network ability, direct connection (e.g. USB) and even RAID features.