Disk enclosure
An external hard drive is a hard drive that can be connected via an interface such as USB, FireWire, eSATA, or Thunderbolt as a removable disk drive directly to a computer. Meanwhile, there are also external hard drives that are connected with a LAN cable or wireless LAN to a network, see Network Direct Attached Storage.
Sizes
The drives external hard drives are identical in construction with internal hard drives. They are connected via a converter electronics (host bus adapter ) for connecting to the common external interfaces. A case is designed to protect against damage caused by electrostatic discharge and very limited for heat dissipation.
Hard disk drives in the form factor 3.5-inch measure approximately 10x15x2, 5 cm. External hard drives with such drives are correspondingly slightly larger. Due to the power consumption of an additional power supply is needed. The maximum capacity of 3.5 - inch HDDs is 4 TB (November 2013)
The smaller 2.5 - inch hard drives have dimensions of about 7x10 cm. The amount is dependent on the capacity and varies between 5 and 15 mm. 2.5 - inch hard drives do not require a separate power supply, but can be supplied via appropriate connections such as USB or FireWire power. Therefore, they are particularly suitable for transporting data (see also sneakers network ). They are equipped with up to 2 TB capacity available (as of November 2013).
Less common is the 1.8-inch format that is now used only for external SSDs that work with electronic memory chips instead of rotating magnetic platters. The 1.0 -inch format has been almost completely replaced by USB sticks.
Meanwhile, there are also external hard drive enclosure with two or four hard drives. This may work for a higher data transfer rate in RAID 0 array or for a greater data security in the RAID 1 array.
For more secure data storage exist accident- only data storage.
Connection types
External hard drives can be connected via various interfaces with other devices. Most common is the USB power supply. The two USB versions 2.0 and 3.0 are in principle compatible with each other, however, the effective data rate for USB 2.0 is on the order of 40 MB / s, while USB 3.0 300 MB / s can be achieved. Since hard drives can achieve data rates of over 100 MB / s, an external hard drive is slowed down by USB 2.0.
In both versions of USB power supply to a 2.5 - inch hard drive via the interface can take place. For USB 2.0, the maximum current is limited to 500 mA, which is why some manufacturers of external hard drives also supply Y cable that allows the flow of a second USB port can be used. With USB 3.0, the current intensity was increased to 900 mA, with virtually any 2.5 - inch hard drive can be operated.
Also when connected via FireWire or Thunderbolt the power supply via the bus can take place. However, with FireWire 400 and 800 achievable data rates are below those faster hard drives, so that this interface is a bottleneck in the transfer. Thunderbolt, however, is fast enough to access to external SSDs in full speed.
The eSATA port provides the even in hard disk drives common bus system S- ATA for external devices. Thus, an external hard drive via eSATA works with the same transfer rate as an internal hard drive that is connected with S- ATA. About eSATA but no power can be supplied so that appropriate external hard drives require power from an AC adapter or a USB port.
Power Supplies
Hard drives usually need 5 to 12 V operating voltage. Initially set the external power supply used to power therefore these two voltages. To connect to the hard drive put every manufacturer at its own plug. Most mini-DIN connections came with 4, 5 or 6 pins are used. The pinouts are different, so that an improper power despite mechanically compatible connector in the worst case can destroy the hard disk connected with reverse polarity. Since the 2010s power supplies with only a voltage and a barrel connector used ( usually 12 V). The 5V required to be generated by a voltage converter in the device.