What are external hard drives and what are their main types?

An external hard drive (also referred to as a portable hard drive) is a data storage device that is externally connected to a PC via USB, eSATA, or FireWire connection. Usually such devices get power directly from the computer to which they are attached via USB (except for 3.5" hard drives typically equipped with an external 12V power supply).

To put it simply, an external hard drive is an additional hard drive you can attach to your PC, whereas an internal hard drive is its essential component that stores all the data, from images and documents to software.

Portable hard drives vary in shapes and sizes. Some are quite large and bulky, while others are not bigger than modern smartphones or tablets. They typically offer more storage space than a USB stick, being at the same time easy to use, compact and portable (at least, most of them).

Last update: December 30, 2021

Time to read: 6 min

What are external hard drives used for?

External hard drives are mainly used as supplemental storages. When your internal hard drive runs out of space, it won’t let you save any more data to it and won’t be able to perform its regular tasks and process data as quickly as it usually does. In other words, it will slow down when you run an application, start playing a game or even browse the Internet. In this case, an external hard drive can provide additional storage space necessary for the computer’s optimal functioning.

On the other hand, external hard drives can be used for storing old photos, videos, documents, and completed projects. Of course, all these files could be stored on your laptop or PC, but if there are plenty of them, they will occupy a substantial part of the internal drive’s space. You may also consider using an external hard drive to store new photos or videos while you're traveling.

Additionally, an external hard drive is a popular solution for data backup and transferring a big amount of information from one computer to another.

Nowadays, there are three main types of external hard drives: HDD (hard disk drive), SSD (solid-state drive) and SSHD (solid-state hybrid drive).

What are the differences between HDDs, SSDs and SSHDs?

A hard disk drive (HDD) looks like an air-sealed case containing one or more platters and an arm with a number of heads (transducers). The data is stored on a platter covered with a magnetic substance and is written and read with the help of heads moving over the platter as it spins. Unlike HDDs, SSDs don't have moving parts. They store data on flash memory chips that are sets of circuits similar to those that are used in USB thumbs. Apart from a memory chip, an SSD has a controller chip that finds the requested data.

SSHDs, in their turn, are traditional HDDs additionally equipped with a small quantity of solid-state storage chips and a controller that decides where some data should be stored: on an SSD or on an HDD.

The internal structure is not the only difference between the drives. Compared to conventional HDDs, solid-state drives (SSD) are more compact and resistant to magnetic fields, accidental drops and other physical shocks, vibrations and extreme temperatures. So, SSDs are considered to be more reliable than HDDs. Additionally, SSDs are faster and have lower power consumption: an HDD with its spinning disks requires much more power when it starts up than a solid-state drive.

As for the capacity, SSDs have up to 4 TB of data storage space, whereas HDDs can store up to 24 TB of data. Yet, when it comes to cost per capacity, HDDs are substantially cheaper, though the prices of SSDs are getting lower year after year.

SSHDs combine the benefits of both types of drives, in particular, a large storage capacity of HDDs and a fast performance of SSDs. In general, an SSHD is slower than an SSD, but faster than a regular hard disk drive. And when you need a big amount of storage (up to 14 TB) at a relatively affordable price (compared to the price of an SSD of the same capacity), an SSHD could be a nice choice.

An HDD is a cost-effective solution for storing a big amount of data you are not going to refer to every day. On the other hand, if you are planning to use an external data storage for photo or video editing and need fast access to the files, an SSD will suit you better.

What other types of external hard drives exist?

Depending on the size and shape (so-called, form factor), external hard drives could be also divided into:

  • 1,8″ – the most compact ones, with up to 1 TB of storage space;
  • 2,5″ – with storage space of up to 5 TB;
  • 3,5″ – with the largest capacity of up to 16 TB, but rather bulky and requiring a network connection or a power supply.

Apart from that, there are regular external hard drives and secure ones equipped with a physical keypad that is used to protect the drive’s data with a PIN.

Typically, external hard drives are connected to a PC via USB, FireWire and Thunderbolt (Apple devices), or eSATA (the least common interface):

  • USB from 1.0 to 4.0, from 12 Mbit/s to up to 40 Gb/s respectively;
  • Thunderbolt (former Light Peak) from 1 to 4 – up to 40 Gb/s;
  • FireWire 400 or FireWire 800 – up to 400Mb/s and 800Mb/s respectively;
  • eSATA – 150 Mb/s or 300 Mb/s.

In addition, HDDs vary depending on their speed rate: from 5,400 RPM (revolutions per minute) to 10,000 RPM.