Portable vs desktop hard drive

An external drive is a hard disk in a box with a USB controller in front of it. Almost everything that separates one external drive from another follows from how big that box is, and how big the box is follows from where it gets its power. Answer the power question and the rest of the choice — capacity, portability, what you can reasonably expect to read on the specification sheet — mostly answers itself.

Power decides the shape

A 2.5-inch portable is bus-powered. The single USB cable carries data and current together, and the drive's motor is built to spin up inside the budget a port can supply. That is why it fits in a coat pocket, why it works plugged into a laptop on a train, and why there is no second cable to lose.

A 3.5-inch desktop unit cannot work that way. Larger platters and a bigger motor need more current at spin-up than a port is obliged to deliver, so the enclosure carries its own mains adapter. The trade is straightforward: it stays where you put it, near an outlet, and in exchange it holds a multiple of what the pocket drive holds.

Those capacity ranges are the ones the catalog actually holds, not a statement about every drive ever sold. They overlap only at the bottom, and the overlap is narrow: past 5 TB, a portable is not an option, and below 4 TB a mains-powered desktop unit is not one either.

Why an external drive lists fewer numbers

Open an internal hard drive's page and you find a cache size and a sequential transfer rate, and on almost every one of them a spindle speed as well. Open an external one and none of the three is there. It is worth being precise about why, because the gap looks like a gap in the research and is not one.

Every specification on this site is read off the manufacturer's own document for that exact product line, and where the document says nothing, the field is left empty rather than filled in with a plausible figure. The fourteen external hard drives tracked here are unanimous: not one of them states a rotation speed, a cache size, or a transfer rate in megabytes per second. Their vendors publish the capacity, the interface and the warranty term, and stop.

There is a reason the vendors stop. A finished external drive is a mechanism plus an enclosure plus a bridge chip, and its throughput is set by whichever of those is slowest at the moment you use it — which is usually the disk, but is a question about the assembled product rather than about the disk. A number on the box would have to describe the pairing, not the drive, and the makers of these fourteen decline to print one.

The consequence runs all the way down into how the site is built. Rotation speed and the two transfer rates are optional fields in the hard-drive schema, because half the category cannot fill them, and an optional field cannot be an axis the comparison ranks drives on — a rank needs a figure both sides are guaranteed to have. What is left is the subject of the next section.

What a hard-drive comparison actually ranks

Put two hard drives side by side here and capacity is the only specification the comparison marks a winner on. Not spin speed, not cache, not the transfer rate — capacity, and nothing else in the specification table. A drive wins that head-to-head by being better on at least one axis and no worse on any other, so with a single axis in play the comparison is saying something narrow and checkable rather than delivering a verdict on the whole product.

That narrowness is honest rather than lazy. Ranking a USB drive against a SATA one on a transfer rate only one of them publishes would be inventing the missing half. Capacity is the figure both halves of the category state, so it is the figure both halves can be ordered on.

Prices sit beside that as their own rows, tracked over time and shown per terabyte as well as outright, which is what makes a 20 TB desktop unit and a 4 TB portable comparable at all despite being different products for different jobs.

Portable SSDs are a different product

Not every drive that plugs in over USB is a hard drive. Portable solid-state drives use the same connector and, unlike the spinning drives above, their makers do publish a sequential rate — the Samsung portables in this catalog state figures from 460 MB/s up to 2,000 MB/s. So "a USB drive has no rate" is true of the external hard drives and false of the portable SSDs, and the distinction is worth keeping straight when reading two listings side by side.

What the two families do share is the enclosure's silence about the rest. No usb-interface entry in this catalog — spinning or solid-state — states a rotation speed or a cache size. A portable SSD has no spindle to rate, and neither kind of enclosure exposes what its controller buffers.

The portable SSDs live with the rest of the solid-state catalog rather than with the external hard drives, so the two do not appear on one page.

Choosing between them

Start from the job. A drive that travels — between an office and a home desk, into a bag, to a client — has to be bus-powered, and that caps you at 5 TB from what is tracked here. A drive that receives a backup every night and never moves has no reason to be small, and the mains-powered units are where the capacity per pound sits.

If you are unsure how much you need in the first place, how much hard drive you actually need works through sizing against the pile you are moving, and covers the recording technology that makes two same-capacity drives behave differently under sustained writes — which applies inside an enclosure exactly as it does inside a case.

Every external drive in our catalog is on one page, and the internal hard drives are on another.