How to choose an SSD

Most SSD advice tours the spec sheet. Four questions decide almost every purchase: what physically fits your machine, how much room you need, how fast the drive has to be, and what a fair price looks like. Answer those four in order and the rest of the sheet is detail.

What fits your machine

Two shapes cover nearly every drive on sale. An M.2 drive is a bare circuit board, a little wider than a stick of gum, that lies flat against the motherboard and is held down by one screw. A 2.5-inch drive is a sealed rectangle that sits in a bay and connects with two cables: a flat SATA data cable, and a power lead from the supply.

M.2 drives come in fixed lengths, and the name states them in millimetres. A 2280 is 22 mm wide and 80 mm long, which is what most desktop boards and full-size laptops expect. Compact laptops and handhelds often have room only for a 2230, at 30 mm long. The screw hole is at a fixed distance, so a longer drive will not mount in a shorter slot.

The trap is that an M.2 slot is a connector, not a promise about protocol. Some slots carry PCIe lanes and speak NVMe. Some carry SATA signalling and speak SATA. Some do both, and on some boards a second M.2 slot shares its lanes with the SATA ports, so filling one disables the other. The notch in a drive's edge connector narrows it down — a drive notched in the M position alone can use four PCIe lanes, one notched in both B and M at most two — but the notch describes the drive, not what the slot behind it will provide.

You can settle all of this without opening the case. Look up your exact model on the manufacturer's support page: the storage section lists how many slots there are, what lengths they take, and whether each one speaks SATA, NVMe, or both. On a self-built desktop the motherboard manual says the same. If a machine has no M.2 slot at all, a 2.5-inch SATA drive is the answer, and it is still a large step up from the hard disk it replaces.

How much capacity you need

Capacity is the biggest lever on price, and the one people shave hardest when the total creeps up. It is also the decision that is most tedious to revisit, because changing it later means moving an operating system or reinstalling one.

Buy for what you expect to hold in two years rather than what you hold today. Operating systems and applications grow between releases, game installs have grown faster than drives have got cheaper, and the folder you promise to clean out never gets cleaner. Through the middle of the range, each step up in capacity usually costs less than twice the step below it, which makes the larger drive the better value per terabyte even though it is the larger bill. That relationship reverses at the top of the range, where the largest capacity on sale carries a premium.

A drive also works better with room to move. Flash cannot overwrite in place: it writes to already-erased blocks and reclaims the old ones in the background, and a drive with little free space has fewer spare blocks to work with. Leaving a slice of a drive permanently empty costs less than the write performance you give up by filling it to the rim.

How fast it needs to be

Two kinds of speed matter, and they behave differently. Sequential work is one large file read or written end to end: copying a video, installing a game, restoring a backup. Random work is thousands of small scattered reads and writes: booting, opening an application, loading a level, building a project.

The number on the box is sequential read, and it describes the rarer case. It is also measured with many requests in flight at once, which a desktop rarely produces. So a drive quoting twice another drive's headline figure is not twice as quick to open your mail client. Everyday responsiveness tracks random performance at shallow queue depths, where the spread between a mid-range drive and a fast one is much narrower than the box suggests.

The interface sets the ceiling. SATA runs at 6 Gb/s, which after encoding leaves a little under 600 MB/s, and essentially every SATA SSD sits at that ceiling — which is why SATA drives are separated by their random performance rather than their sequential numbers. NVMe drives talk over PCIe lanes instead, and each PCIe generation roughly doubles what a lane carries, so the ceiling rises with the generation of both the slot and the drive.

Generations are compatible in both directions. A newer drive in an older slot runs at the older slot's speed; an older drive in a newer slot runs at its own. That makes PCIe generation a question about money rather than compatibility. You are buying headroom, and it only pays back if you move large files often.

Does the flash type or DRAM actually matter

Flash cells store bits as voltage levels, and how many bits share a cell is the main dividing line between drives. TLC holds three bits per cell; QLC holds four. Packing more bits in makes each cell cheaper per gigabyte, and also slower to write and shorter-lived, because the voltage bands are narrower and there are more of them to tell apart.

Both hide this behind a cache. A slice of the flash is operated at one bit per cell, where it is much faster, and incoming writes land there before being folded into their denser final form later. This SLC cache is why a short copy is quick on almost any drive, and why a long one is not: once the cache is full, the drive falls back to writing at its native rate, and that drop is steeper on QLC than on TLC.

The other part worth knowing about is the DRAM chip some drives carry. It holds the table mapping logical addresses to physical flash locations. A drive without one keeps that table in the flash itself, or borrows a small window of the host's memory, and pays for the lookup on scattered access. The penalty grows with capacity, because the table does.

The honest summary is that for a boot drive doing ordinary desktop work, none of this is visible. It surfaces when you write tens of gigabytes in one go, when a drive is close to full, or when something hammers it with small writes for a long time. If none of that is you, put the difference into capacity instead.

What a fair price looks like

The figure that compares drives honestly is price per terabyte: divide the price by the capacity, and a 500 GB drive and a 4 TB drive land on the same scale. It is arithmetic rather than a benchmark, and it takes a few seconds.

Do it within a capacity tier before you do it across tiers. Tiers move independently, because flash supply and the mix of parts a maker has to shift push one size down while another sits still. A strong price on 1 TB tells you nothing about whether 2 TB is well bought this week.

That is also why a percentage off a list price means very little here. What matters is the price against what this drive has actually been selling for lately. How we score deals sets out how we measure that, including the cases where we decline to show a score at all.

What can we compare, and what can we not see

We compare SSDs on three figures: capacity, sequential read, and sequential write. Manufacturers state all three for every drive they sell, at every tier, so comparing on them compares like with like. Sequential write is part of that comparison, but the guide does not walk through it on its own the way it does capacity and sequential read.

One model is usually sold in several sizes at quite different prices, so capacity is the axis we split variants of one drive on. The 1 TB and the 2 TB version of a model are separate entries with separate price histories, rather than one entry showing a range that flatters the smaller size.

Then there is what we cannot see. Whether a drive carries DRAM, which NAND it uses, and what endurance it is rated for are absent from most listings we ingest, so we do not filter on them. Where a listing does not state a value, the field stays empty rather than being filled in from a model number, because two drives sold under one name can differ inside. An empty field means the listing was silent, not that the drive lacks the part.

How do you narrow it down to one drive

Answer the four questions in order and the shortlist mostly writes itself. Confirm what your machine takes, and let that rule out everything that will not fit. Pick the capacity you want to be living with in two years. Decide whether you need sequential speed at all, or just a drive that is not a hard disk. Then compare price per terabyte inside that capacity, and buy on the price rather than on the size of the discount.

When you are ready, every SSD we track is on one page, and the guided pick puts the same listings in order for you.