EMZETT.External Hard Drives
In short: Mass storage in its own enclosure, connected to a computer via USB (or, more rarely, network) — practical for backups or transporting data between devices.
In more detail: Internally, this is usually a normal HDD or SSD, connected via USB bridge electronics. The transfer speed depends both on the internal drive and on the USB standard used.
In Depth
Portability as the main advantage
The main advantage of external hard drives lies in their portability and easy handling, without tools or opening up the system: plug in via USB, automatically recognised by the operating system, immediately usable — unlike internal hard drives, which need to be mounted in the case and permanently wired. This flexibility makes them ideal for backups, transporting large amounts of data between devices, or as additional storage for laptops with limited internal space.
The speed chain
The actual speed of an external hard drive is limited by several factors that have to be considered separately: the internal drive itself (an external HDD is fundamentally slower than an external SSD), the USB bridge electronics in the enclosure (which translates the internal SATA/NVMe interface to USB), and the USB standard and cable used. A fast internal NVMe SSD in an external enclosure, for example, doesn’t help if the bridge or the computer’s USB port only supports older, slower USB generations — the slowest part of the chain always determines the overall speed.
Form factors and power supply
External hard drives also differ in construction: 2.5-inch enclosures (typical for laptop drives and external SSDs) usually don’t need a separate power supply and draw their power directly from the USB port, while larger 3.5-inch enclosures (common for high-capacity HDDs) usually need their own power supply because of higher power consumption. This makes 2.5-inch drives considerably more travel-friendly, even though at very large capacities (several terabytes) they’re often more expensive per gigabyte than 3.5-inch HDDs.
Backup strategies
External hard drives are a central building block of common backup strategies, such as the “3-2-1 rule”: at least three copies of the data, on at least two different storage media, at least one copy at a physically separate location (e.g. in case of a fire or theft of the original devices). An external hard drive alone at the same location as the PC protects against internal drive failure, but not against events affecting both devices at once.
See also: Internal Hard Drives, USB, SSDs, HDDs