An external hard drive is a hard disk which is meant to be placed outside of the computer case. This allows for additional storage in situations where placing a drive in the computer itself is either not possible or undesirable.

External hard drives can either be made by a manufacturer or created by putting a standard hard disk inside an external drive enclosure. This external enclosure provides protection to the drive circuitry while also suppling the data connections and power.

Connections

There are three major types of connections which are utilised to connect external hard drives.

• Native Drive Protocols
• Direct Attach Serial Protocols
• Network Protocols

Native Drive Protocols

SCSI, SAS, and SATA protocols can be used to irectly connect the hard drive to an internal disk controller, without the need for any intervening controller.

Direct Attach Serial Protocols

USB or FireWire connections are typically used to attach consumer class external hard drives to a computer. Unlike SCSI, SATA, or SAS these require circuitry to convert the hard disk's native signal to the appropriate protocol. IDE hard disks use this solution most frequently because of the relative low cost and high availability of this technology.

Network Protocols

iSCSI, NFS, 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.

Src Wikipedia

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