There are standards that allow additional hard drives to be connected to computers without being inside the case; these drives are called external hard drives and portable hard drives. Although external hard drives were first sold to consumers in the early 1980s -- most computers didn't have internal drives then -- the didn't take off until the early 2000s, several years after the USB standard was released. The primary difference between them is how they connect to a computer, and whether or not they can be unplugged and easily carried.
An enclosure is what keeps the hard drive from rattling around, being knocked off the desk or shelf, and keeps it stable while operating. Enclosures may, or may not, provide a jack for plugging in an AC adapter to power the drive, or have their own electrical plugs. One of the differentiators between portable hard drives and external hard drives is that the enclosures are more collision and shock resistant, and are usually self-contained in terms of power -- most portable hard drives are built around 2.5-inch diameter laptop hard drives, and powered directly by USB or FireWire cables. External hard drives have larger enclosures, may contain multiple drives in a RAID configuration, and usually draw their power from a wall socket or AC adapter, rather than from the computer they're connected to.
The vast majority of portable hard drives use USB as their connection protocol, because most PCs have a USB port, and the port can provide enough power to run the drive. Some portable hard drives, especially those used by professional videographers, use FireWire as their connection method. FireWire doesn't require an intermediate computer between devices. By contrast, most external hard drives connect to a network and are visible to all computers on the network, or they connect by the eSATA standard, to give faster data transfer rates. Some portable hard drives can connect to computers wirelessly, through Wi-Fi, though most do not – nearly every other connection mechanism is much faster.
Portable hard drives are most often used for backups between computers, and for moving very large files – though the increasing size of USB Flash drives has made this second purpose less common. External hard drives are often used for centralized backups of entire networks, or network file share servers, where companies share assets -- like art files or animations -- used by several departments. In home networks, external hard drives get used as network accessible storage, and hold music and multimedia files that can be accessed by any computer in the house.
Hard Drive Capacity and Prices
The relative pricing on external hard drives and portable hard drives is driven by how quickly the hard drive can find, read and write data, how quickly it can transfer the data to the computer, and hard drive size. For portable hard drives, there is a small premium to be paid for the 2.5-inch platter size, and for both types of drives, solid-state models exist that are blazingly fast, and high-priced to match.
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