Six Things Everyone Must Know About External Hard Drives

An external hard disk is a hard disk drive that can be connected directly to a computer via an interface such as USB, FireWire, eSATA, Thunderbolt or as a removable disk drive. There are also external hard drives that connect to a network with a LAN cable or wirelessly via WLAN, see Network Direct Attached Storage (NAS). There are also external “hard disks” that are not actually magnetic disk storage drives – i.e. hard disk drives – in the true sense of the word, for example external flash storage such as SSDs.

The drives of external hard disks are identical in construction to internal hard disks. They are connected via converter electronics (host bus adapters) to connect to common external interfaces. An enclosure is used to protect against damage caused by electrostatic discharges and, to a very limited extent, to dissipate heat.


In the following, only hard disk drives (abbreviation HDD) are described, in which the data is stored on rotating, magnetic metal disks. Solid-state drives, abbreviated SSD, which are an immovable block, are to be distinguished from this. In terms of data security, interfaces and power consumption, both are equivalent. SSDs are more resistant to shock, quieter and faster, but cost many times more than HDDs for the same capacity, so they are hardly used in the field of external hard drives.

HDD sizes

HDD drives in the 3.5-inch form factor measure around 10 cm × 15 cm × 2.5 cm. External hard disks with such drives are correspondingly somewhat larger. Due to the power consumption, an additional power supply is needed.

The smaller 2.5-inch hard drives have dimensions of about 7 cm × 10 cm. The height depends on the capacity and varies between 5 and 15 mm. 2.5-inch hard disks do not require their own power supply unit, but are supplied with power via suitable connections such as USB or FireWire. Therefore, they are particularly suitable for transporting data.

Less common is the 1.8-inch format, which is still partly used today for external SSDs that work with electronic memory modules instead of rotating magnetic disks. The 1.0-inch format has been displaced by USB sticks.

External hard drive enclosures with two or four hard drives also exist. These can work in a RAID 0 array for a higher data transfer rate or in a RAID 1 array for greater data security.

Accident-proof data storage units exist for particularly secure data storage.

Connection types

External hard disks can be connected to other devices via various interfaces. The most common connection is via USB. The two USB versions 2.0 and 3.0 are basically compatible with each other, but the usable data rate for USB 2.0 is in the order of 40 MB/s, while around 300 MB/s can be achieved with USB 3.0.

With both USB versions, a 2.5-inch hard drive can be powered via the interface. With USB 2.0, the maximum current is limited to 500 mA, which is why some manufacturers of external hard drives include Y-cables with which the current of a second USB port can be used. With USB 3.0, the current has been increased to 900 mA, with which practically every 2.5-inch hard drive can be operated.

Power can also be supplied via the bus when connecting via FireWire or Thunderbolt. However, the data rates achievable with FireWire 400 and 800 are lower than those of fast hard drives, so this interface is also a bottleneck in transfer. Thunderbolt on the other hand is fast enough to also access external SSDs at full speed.

The eSATA port, which is no longer in use, also made the SATA bus system, which is common for hard drives, available for external devices. Thus, an external hard drive via eSATA works with the same transfer rate as an internal hard drive connected with SATA. However, there is no power supply via eSATA, so corresponding external hard disks need power from a power supply unit or a USB port.

Power supplies

3.5-inch IDE/PATA hard disks usually require 5 and 12 V operating voltage. Initially, the external power supplies used for supply therefore provided these two voltages. However, each manufacturer relied on their own connectors to connect to the hard drive. Mini-DIN connections with four, five or six pins were usually used. The pin assignments also differ, so that an incorrect power supply can destroy the connected hard drive in an unfavorable case due to over-voltage or polarity reversal despite a mechanically suitable plug. Since the 2010s, power supplies with only one voltage and a hollow plug have been used (mostly 12 V). The required 5 V are generated in the device by a voltage converter.


The stored data or the entire data storage of a hard disk can be completely or partially encrypted by means of software- or hardware-based hard disk encryption. This applies to internal and external hard disks, but is particularly important for external hard disks to prevent unauthorized access to data in the event of loss.


In order to automatically back up to an external drive or from an external drive, you could use a backup solution, such as BackupChain.