Shopping for an SSD means facing a sea of acronyms. The trickiest of them are jagged reefs sticking out of the water, shaped like circuit boards and ready to run your upgrade aground if you’re not careful.
“SSD,” standing for “solid-state drive,” is the one you probably know. An SSD is a storage drive made up purely of flash memory in modules called “NANDs,” and governed by a controller chip. Simple enough—but there is much complexity beneath the surface of those three letters.
SSD navigation has gotten even more treacherous in the last few years, with the emergence of three key new things you need to know about when shopping for an SSD: M.2, PCI Express (or “PCIe”), and NVMe. All three are centered around making SSDs smaller or faster. They also make buying an SSD more complicated than ever.
SSDs: Why They Are Changing Shapes
Until recently, the typical SSD was a little slab, designed to fit into the same space in a laptop or PC that a hard drive would. (The lingo was “2.5-inch drive” for that size of SSD.) That’s changing nowadays.
Almost all new desktop-PC motherboards, and the logic boards on many new laptops, incorporate slots meant for much smaller SSDs. That’s especially important in thin laptops, where interior space is so scarce. These slots are known as M.2 slots, and they accept M.2 SSDs that look like sticks of silicon chewing gum. Just about every new desktop motherboard these days has at least one such slot; some have two or three. And depending on the drive, that slim little stick of an SSD may be much faster than those bigger drives you are used to.
Why did SSDs take so long to get so small? That’s a good question, with a simple answer. SSDs never needed to be that big in the first place, from a strict manufacturing point of view. “Classic” 2.5-inch SSDs have a lot of dead space inside, but they were designed that way to fit into existing bays in laptops and desktop PCs. In slimming down laptops and tablets to extremes, one thing came clear: That much-fatter kind of drive would have to go. An M.2 SSD reduces the SSD to its essentials: just a strip of a circuit board, studded with chips. Versus SSDs of the past, M.2 is much leaner and much easier to fit into tight spaces.
First, a Bit About M.2
Most M.2 drives aren’t pretty; they tend to look like bare circuit boards with various silicon chips grafted on them, including the NAND modules that store your data. (Check out our guide to SSD lingo at Buying a Solid-State Drive: 20 Terms You Need to Know.) Some may be topped by a heat spreader or heat sink that is usually equal parts practical and decorative. The most important thing to know about M.2, though, is what it is and what it isn’t.
Although M.2 is commonly referred to as an interface, that’s not the whole story. M.2 is also a shape or form factor of drive, though M.2 also governs the keying that lets a drive fit onto a motherboard.
The data bus, or pathway, over which your data travels to and from an M.2 drive is a whole other matter, and it takes one of several forms. And that is where NVMe comes in. We’ll get to the significance of NVMe in a moment; first, let’s discuss the key physical traits of an M.2 drive that you need to understand. (The video below is also a good primer.)
As we discuss in our parallel roundup, The Best M.2 Solid-State Drives, M.2 drives are differentiated by a four- or five-digit number listed in their specifications or names. The number is a measurement. It’s given in millimeters, with the first two digits being the drive’s width, and the second two or three digits telling you how long it is.
In practice, all of the upgrader- or PC-builder-minded M.2 SSDs and slots we have seen to date have been 22mm wide, so you can expect this number to start with “22.” The most common lengths are 80mm (“M.2 Type-2280”) and 60mm (“M.2 Type-2260”). Drives as short as 42mm (“M.2 Type-2242”) or as long as 110mm (“M.2 Type-22110”) do exist, though. Why the differences in length? The longer the drive’s PCB, the more surface area it will have onto which chips can fit.
Length mostly matters if fitting an M.2 drive into a laptop. Most desktop motherboards with M.2 slots have mounting points for multiple lengths of drive (usually, 80mm, 60mm, and 42mm, and sometimes 110mm), whereas most laptops fit just one size. Check the space available before you shop.
The length of an M.2 drive doesn’t always correlate 1:1 with drive capacity, but the bigger the stick, the more memory modules engineers can stuff onto a PCB of a given size, all else being equal. Because of space and density limits, most M.2 drives to date have topped out at 1TB, though 2TB, 4TB, and even 8TB M.2 SSDs are now seeing the light of day. You’ll see four broad classes of capacity on most M.2 SSDs, with the capacities varying within the class according to how much data the drive maker has put aside for “overprovisioning” (essentially, a safety margin put aside for when the drive ages and some cells fail). These capacity classes are…
- 120GB or 128GB
- 240GB, 250GB, or 256GB
- 480GB, 500GB, or 512GB
- 960GB or 1TB
Now, let’s reiterate an important point: A drive may be an M.2 stick of whatever length and capacity, but that doesn’t tell you about the bus that it makes use of. That detail is vital to know—just as important as making sure the drive’s length works in the space you have.
What’s the Bus? PCI Express and NVMe
The first M.2 drives were Serial ATA (SATA) drives, in essence a bare version of their chassis-enclosed 2.5-inch kin. You can still readily find SATA-bus SSDs in the M.2 shape. They are common, and most M.2 slots will accept them. In some cases, both 2.5-inch and M.2 versions of the same drive are available, with little difference in performance between them. (Take a look at our legacy reviews of the SSD 850 EVO 2.5-inch and the SSD 850 EVO M.2 for an illustration of that.) That’s because, with any SATA-at-heart SSD, your data travels the same paths whether it is a big 2.5-inch SSD connecting to your PC over a classic SATA connector via a cable, or an M.2 stick in an M.2 slot.
SATA-based M.2 SSDs are all well and good, but PCI Express is where the cutting-edge speed is. Your system specifically needs to support PCI Express on its M.2 slot to use these drives; some desktop boards support both kinds. A given laptop might support only M.2 SSDs that use the SATA bus, and that limits what you can do in terms of upgrades. The only reason you’d upgrade the drive, in that situation, would be to boost the available storage capacity.
Some premium laptops can make use of PCI Express M.2 drives. (Note that some, like the latest Apple MacBook Pros, have PCI Express drives soldered, in not-upgradable fashion, to the laptop’s mainboard. “PCI Express SSD” doesn’t necessarily mean “removable M.2 SSD module.”) And as mentioned, almost all new desktop mainboards now have M.2 slots, and most now support PCI Express M.2 SSDs.
The first generation of M.2 PCI Express SSDs made use of a PCI Express x2 interface, which defines a throughput ceiling that’s higher than SATA 3.0’s, but not enormously so. That’s changed. Today’s mainstream M.2 drives support PCI Express 3.0 x4 (four lanes of bandwidth), working alongside a technology called Non-Volatile Memory Express (NVMe). The idea behind NVMe is to accelerate performance further, especially with hardy workloads.
NVMe is a control protocol for SSDs that has emerged in the last few years. You may have heard of the term “AHCI” in passing over the years; it is the control scheme used by hard drives and SATA SSDs for data flow over the SATA bus. AHCI was designed back in the days when hard drives were king, though it does work with SSDs. But the concepts underlying it were conceived back when storage overwhelmingly meant spinning platter mechanisms. NVMe, in contrast, is designed from the ground up to govern solid-state memory, and it is optimized for the flash-based realm. It’s designed to replace AHCI in the latest SSDs.
NVMe is the buzzword to look for in M.2 SSDs, but know that your system and its motherboard specifically need to support PCI Express NVMe drives in the BIOS for the drive to act as a bootable device. Motherboards based around AMD’s and Intel’s last few chipsets support PCI Express x4 NVMe M.2 drives, but you’ll want to check on a board-by-board basis. Some desktop boards now have two or more M.2 connectors, a trend we’re seeing more and more with higher-end, late-model mainboards. But outside of desktop motherboards from the last two or so years, NVMe-capable M.2 slots are not a given. So consult your manuals closely before buying one of these drives.
Also, make sure that if you are looking for an NVMe drive, and your system supports it, that any PCI Express drive you are looking at is specifically an NVMe model. That a drive uses the PCI Express bus is not a guarantee of that; PCI Express M.2 SSDs existed before NVMe, and some of them are still on the market. PCI Express x4 NVMe M.2 drives are now the most common kind, though, and are impressively fast drives, leaving the fastest SATA-based drives far behind. Samsung, among the current top dogs in SSDs, was an NVMe pioneer, but most other SSD vendors have gotten into the NVMe game at this point, too, including ADATA, Corsair, Crucial, Kingston, Seagate, and WD.
Just know—we’ll reiterate this for emphasis—that you need the right motherboard-level support to install one of these drives. For desktop PCs, your motherboard specifications are the place to check. Many recent M.2-equipped mobos support both bus types of M.2 SSD (M.2 SATA and M.2 PCI Express/NVMe). For a laptop, it may require pinging the support team of the laptop maker to find out what is inside (assuming you can even get inside the laptop).
Also, as we mentioned earlier, realize that a select few laptops have begun soldering their storage directly to the mainboard to save further space, so a drive upgrade may not be possible at all. And recognize that upgrading a laptop to a new SSD may violate the terms of any existing warranty. Check.
PCI Express 4.0: Next-Generation SSDs
You’ll also see, when shopping for M.2 PCI Express drives, a few cutting-edge (and usually slightly pricier) drives that mention support for PCI Express 4.0, as opposed to just 3.0.
We’ve tested a handful of M.2 SSDs that support this newer 4.0 version of the PCI Express bus, and they are fast indeed. How much you’ll be able to tell the difference, though, comes down to what you do with your PC.
The read speeds for a few (such as Samsung’s flagship SSD 980 Pro) are rated as high as 7,000MBps. These drives are a peek at the future, but for the moment PCI Express 4.0 is supported only on desktop systems employing the AMD X570 and B550 chipsets (for mainstream Ryzen CPUs) and the AMD TRX40 (for third-generation Ryzen Threadripper ones). You can use these PCIe 4.0-based in Intel-chipset systems, but they’ll just bounce down to slower PCIe 3.0 speeds.
Should you opt for one? If you’ll be upgrading a late-model AMD PC with the right chipset, sure. You could also consider one for future-proofing if not, but by the time you upgrade again, prices could well have fallen relative to 3.0 models. At the moment, we’d leave them to dedicated speed freaks, hardcore gamers, and content creators working on today’s AMD platforms.
NVMe in Other Shapes: PCI Express Expansion Cards and U.2 Drives
If you’re looking to add a PCI Express/NVMe drive to a desktop whose older motherboard lacks an M.2 slot, one option is an M.2 drive on a “carrier card.” In essence, the M.2 drive gets fitted onto a PCI Express expansion card that has an M.2 slot aboard and drops into an ordinary PCI Express slot of at least four lanes.
We’ve seen solutions like this from Intel, Gigabyte, Plextor, Kingston, and others. Also, in a few one-off instances, some motherboard makers (such as Asus) bundle an empty M.2 carrier card in the box with their high-end mainboards. With one of these, an M.2 drive on a PCI Express expansion card lets you tap the speed of PCI Express/NVMe without having a supporting M.2 slot. The card may also add bootability, so check for that.
Some PCI Express M.2 SSDs can run hot under sustained use, so having the M.2 module mounted on a vertical card can also mean better ventilation, and, in theory, less or no throttling due to heat. That said, these drives are so fast that under most normal use they get their transfers done before heat can become a major issue.
One other form that NVMe drives take is confined at the moment to a couple of Intel SSDs. The Intel 750 Series SSD that we tested in 2015, as well as the late-2017 Intel Optane SSD 900P Series, are NVMe drives that come in two forms. One is a straight-up PCI Express card; the other form factor is a large 2.5-inch drive that resembles a chunky hard drive with a weighty heat sink aboard. It uses a server-minded physical interface known as U.2, which is seldom seen in consumer drives. A few high-end motherboards, such as the Designare series from Gigabyte, may have an onboard U.2 port, but most desktop motherboards will need a specialized adapter for U.2 that plugs into an M.2 slot.
NVMe Drives: Shopping Tips
SIZE IS KEY. We explained earlier the secret code to decipher the length and width of an M.2 drive. Make sure the length matches the available space for the drive. (This matters most with laptop upgrades.) Most will be 60mm or 80mm long. Also look at any heat sink or spreader atop the drive, and make sure it will not interfere with the upgrade. Often, you can remove the spreader, but it’s there for a good reason if it is present. Just know that in, say, a laptop upgrade, an M.2 PCI Express drive adorning a big heatsink like the one on the TeamGroup T-Force Cardea will not fit unless you remove that hardware.
BUS DETAILS (CAN BE TRICKY). With laptops, an SSD upgrade is often a binary choice that hinges on the kind of SSD supported. (Does the laptop support SATA, or PCI Express? End of story.) The usual scenario there is that you are swapping out one M.2 drive for a more capacious one. Most laptops don’t have spare M.2 slots left empty, so the decision path there is pretty straightforward.
Desktop motherboards are more complicated, in that some support both SATA-bus and PCI Express-bus M.2 on a given slot. In boards that have two M.2 slots, the support may vary between the two. Others support only SATA; still others support only PCI Express. (And, in the case of PCI Express M.2, an older motherboard may not support PCI Express x4, only x2!) In short: It’s a swamp to be navigated with care. You need to know exactly what your board is optimized to use and buy accordingly.
WATCH THAT BOOT. If your desktop is getting a PCI Express/NVMe drive for the first time, verify with the motherboard or PC maker that this kind of drive will be bootable. It’s unlikely, but a BIOS upgrade may be needed to get you there. It’s more an issue with older motherboards than current ones.
PRICE IT OUT PROPERLY. Comparing SSDs in terms of value for money is tricky, but the best metric is the cost per gigabyte. PCI Express-bus models do command a premium. Divide the price (in dollars) by the capacity (in gigabytes) to get the cost per gig. For example, a 1TB drive that’s selling for $100 works out to about 10 cents per gigabyte. That’s a ruler you can use to measure drives of different capacities against each other.
So, Which PCI Express SSD Should I Buy?
Okay, let’s get shopping with our favorites below. Also note: For a guide to all of the best M.2 drives we’ve tested (SATA and PCI Express/NVMe together), see our roundup of the best M.2 solid-state drives. If you’re also interested in factoring in 2.5-inch SATA drives, check out our guide to the best internal SSDs overall, which includes all three types.
You can also check out our roundups of the best budget SSDs, as well as our best external hard drives for Mac and our overall best external hard drives.
