External hard drives

Drives sold in an enclosure, where the specifications describe the box as much as the drive.

An external drive is a bare drive plus a USB bridge, and listings often describe the bridge rather than the platters — an interface rate in place of a transfer rate, and rarely a spindle speed at all. Where a figure here is missing it is usually because the listing never stated it, which is worth knowing before comparing one against a drive that did.

What an ordinary external drive looks like

Medians, with values beyond 1.5× the interquartile range excluded so one listing cannot move them. A class holding fewer than 8 drives publishes no baseline at all.

How this class is distributed

72 drives in this corpus sit in this class. By capacity:

The listing describes the enclosure, not the drive

An external drive is a bare drive with a USB bridge in front of it, and manufacturers specify the bridge. That is why transfer figures here are often an interface rate rather than anything the platters can sustain, and why spindle speed and cache go unstated far more often in this class than in any other. A missing figure on this page is almost always a listing that never published one, which is worth knowing before comparing an external drive against an internal one that did.

Half of these are laptop drives in a box

This class splits roughly evenly between 3.5-inch drives, which need mains power, and 2.5-inch drives, which run from the USB port. The distinction decides whether the thing is portable in any real sense, and it is frequently not stated — the clue is usually the power supply in the photographs rather than anything in the specification table. It also explains the spread in this class: the 2.5-inch half inherits the capacity ceiling and the higher price per terabyte of laptop drives, and the 3.5-inch half does not.

Shucking, and why the price per terabyte looks odd

External drives are sometimes cheaper per terabyte than the equivalent bare drive, because the enclosure is sold as a consumer product and the drive inside it is not always the model the price would suggest. Buyers remove the drive from the case for this reason. It usually voids the warranty, the drive inside is not guaranteed to be any particular model, and some enclosures use a bridge that formats the drive in a way an ordinary controller will not read. It is a real practice and a real gamble, and the price per terabyte on this page is the number that makes people attempt it.

What to check before buying one

The useful questions are whether it needs mains power, what is actually inside it, and what the warranty is — and only the first is reliably answerable from a listing. Where the drive will be moved regularly, a 2.5-inch bus-powered unit is the only sensible choice. Where it will sit on a desk permanently, a 3.5-inch unit is cheaper per terabyte and generally faster, and the portability that the class name implies is not being used anyway.

Shingled recording, and the word that is missing

Drives record in one of two ways. Conventional recording writes tracks side by side; shingled recording overlaps them like roof tiles to fit more on a platter, which makes rewriting a track require rewriting its neighbours. On sustained writes a shingled drive can slow to a fraction of its rated speed once its cache fills, and in a RAID rebuild that can turn hours into days. The tell is what listings choose to say: in this corpus the term CMR appears roughly twenty times more often than SMR, because conventional recording is advertised as a feature and shingled recording is simply not mentioned. Silence is not evidence of either, but a drive that makes a point of saying CMR is telling you something a drive that says nothing is not.

Recertified and renewed listings

Roughly one listing in seven here is sold as recertified, renewed or manufacturer-refurbished. These are drives that have been returned or pulled from service, tested and resold, and they are usually the cheapest per terabyte on the page. What changes is the warranty — often months rather than years — and what is not disclosed is the hours already on the drive. For storage where the data exists elsewhere, the arithmetic can be sound. For the only copy of anything, buying a drive with an unknown history to save a fraction of its price is a poor trade, and the price per terabyte on this page does not distinguish between the two.

Why the capacity you get is smaller than the one you bought

Drive manufacturers count a terabyte as a trillion bytes; operating systems generally count it as 2^40, about ten per cent more. A 4 TB drive therefore shows as roughly 3.63 TB before a single file is written, and nothing is wrong. Formatting and the file system take a further small share. This is not a specification any listing states and it is the most common reason a new drive appears to have arrived short, so it is worth expecting rather than discovering.

One external drive is one copy

The common practice is to buy an external drive, copy everything important onto it, and treat the problem as solved. It is not: the drive is a single point of failure sitting in the same building as the machine it is protecting, exposed to the same theft, fire and flood. The usual rule is three copies on two kinds of media with one somewhere else, and an external drive is a good way to hold one or two of those. Where it is the only copy of something irreplaceable, the honest description is that the data is now on a portable device with a one-year warranty.

Reading these figures

Every number above is a median of what manufacturers publish, not a measurement we took. Where a specification is missing from this table it is because too few listings in the class stated it for a median to mean anything — that is a fact about the listings, not about the drives.

Narrow this class by capacity and budget in the hard drive finder.

Computed from the corpus on 2026-08-29.