If you've read through this whole buying guide and have a particular port or slot not covered yet, that's because you probably have one of the two outlier ports installed in your system: U.2 or mSATA.

U.2 is rare in consumer PCs; it's mostly made with enterprise customers in mind. A U.2 drive like the now-vintage Intel SSD 750 Series connects to a U.2 port on the motherboard via a special cable, or to a PCI Express M.2 slot using a special adapter. These drives almost always come in the 2.5-inch form factor. Unless you have a U.2 port on your desktop motherboard you want to use, you can ignore them. (And even if you do, you can still probably hook up an M.2 drive.)

mSATA, short for mini-SATA, is a predecessor to the M.2 form factor. It was primarily built into laptops, though some older desktop motherboards may have an mSATA slot aboard. With mSATA, the slots and drives use only the SATA bus, unlike M.2's SATA and PCIe support. For all intents and purposes, mSATA is a dead end, though you might run into it if you have an older laptop or desktop.

Last of all is the "AIB SSD" mentioned earlier, which comes on a PCI Express expansion card, much like a small graphics card. Most use the HHHL (half-height, half-length) form factor, letting them fit into compact, low-slung PCs, and plug into the same PCIe slots you'd plug any other expansion card. You'd only want to opt for one of these, though, on a desktop PC that lacks an M.2 slot or a SATA port/drive bay you can use.

Let's get into the issue of bus type in a little more depth. Oftentimes, you won't have a choice of what bus variety you need. But you need to know some background to figure out what you have and what you should buy.

Serial ATA (SATA) is both a bus type and a physical interface. SATA was the first interface that consumer SSDs used to connect to motherboards, like the hard drives that preceded them. It's still the primary cable-based interface you'll see for 2.5-inch solid-state drives.

The SATA interface is capable of sequentially reading and writing a theoretical maximum of 600MBps in an ideal scenario, minus a bit for overhead processes. Most of our testing has shown that the average SATA drive tops out at roughly 500MBps to 550MBps; in sequential tasks, the real-world difference between the best SATA drive and a merely average one is pretty small.

However, there's also the matter of 4K random read and write performance speeds to consider. These speeds reflect how quickly the drive performs in day-to-day tasks. (Think booting Windows, launching games, loading levels in those games, or working in applications like Adobe Photoshop.)

For most gamers and general users, 4K random read/write speeds are going to determine how much you actually feel the "speed" of a drive. They should be the most important spec to keep in mind if you plan on turning your next SATA-based SSD into a boot drive or backup storage for your trove of games or creative projects.

SATA-based SSDs have shown that in 4K random read and write, specifically, SATA isn't quite out of the game yet, offering performance in loading games or applications that's on par with...

The original implementation of the PCI Express interface for SSDs took the form of cards that occupied one of the PCIe slots on a desktop motherboard, and you can still find carrier cards that let you plug M.2 drives into a standard PCIe slot. Nowadays, though, the most popular PCI Express SSDs mount into an M.2 slot, though as we said above, you should make sure that your M.2 slot (assuming you have one in the first place) supports PCIe drives before you make your purchase. Some support only the SATA bus; some support PCIe only; and some support both.

A further wrinkle around the PCIe bus: All recent drives and slots support a transfer protocol known as NVMe (for Non-Volatile Memory Express). NVMe is a standard designed with flash storage in mind (opposed to the older AHCI, which was created for platter-based hard drives). In short, if you want the fastest consumer-ready SSD, get one with NVMe in the name. You'll also need to be sure that both the drive and the slot support NVMe. (That's because some early M.2 PCIe implementations, and drives, supported PCIe but not NVMe.)

Then there's the difference between PCI Express generations. As you'd expect, drives speed up through each successive generation. PCIe 4.0 set peak-sequential speed records for consumer storage, and the first PCIe 5.0 drives have predictably blown these records away. PCIe 4.0 requires support from the specific desktop or laptop platform. PCIe 4.0 came to market with third- and fourth-generation Ryzen processors from AMD, and PCI Express 4.0 support is now available on the Intel side with Intel 500 Series chipset and later platforms with 11th Gen or higher CPUs on the desktop. (It's also part of the company's mobile chip platforms from the 11th Generation onward. Indeed, the very latest desktop Intel platforms support the emerging PCIe 5.0, whose system requirements are more onerous than PCIe 4.0.)

Most folks won't need (or indeed even be able to leverage) the speed of PCIe 5.0. On the market, you will find two main iterations of PCI Express drives in production right now: PCIe 3.0 x4, and PCIe 4.0 x16. (The "x" in each of these naming schemes refers to how many lanes the drive has available to transfer data.) PCIe 3.0 x2 drives exist, but avoid them at this point. A mainstream choice is a PCIe 3.0 x4 drive; you'll want a faster PCIe 4.0 model if you have a AMD Ryzen-based desktop based on the X570, B550, TRX40, or later chipsets, or an Intel-based desktop with an 11th Gen or newer processor. Either way, you need a motherboard that explicitly supports PCIe 4.0 or 5.0 on an M.2 slot. (Check the specs for PCI Express 4.0 support, and on which slots, before you dive in.)

Even PCIe 3.0 is significantly faster than SATA in straight-up sequential tests, though. But that's just sequential speeds, and how fast a drive can copy a folder from one part of itself to another isn't all that matters these days. There's also the issue of capacity,

When an SSD manufacturer advertises the speed of a particular drive, it will usually be shown in one of two ways: the maximum theoretical sequential read/write speeds (expressed in megabytes per second), or the maximum theoretical random—or "4K," as in four-kilobyte blocks—read/write speeds (expressed in IOPS or input/output operations per second). In practical terms, however, 4K read/write results can be expressed just as easily in MBps.

Sequential write speeds are generally (though not always) tied to the results you can expect while transferring large singular files (think of a high-resolution movie or an ISO optical disc image), while 4K read/write results are more reflective of things like game loading times or how quickly your operating system can fetch files.

The maximum sequential read speed that's theoretically possible for a SATA drive is 600MBps, though as we said above, we haven't seen any drives reach that limit even in ideal testing conditions. The theoretical peak sequential read speed for PCI Express 3.0 x4 drives is much faster—3,940MBps, although the fastest one we've tested in-house is the Samsung SSD 870 EVO, which topped out at 3,372MBps read speed in the Crystal DiskMark 6 benchmark.

As mentioned earlier, PCI Express 4.0 is faster still, but it requires a late-model AMD or Intel platform with PCIe 4.0 support in the chipset and on an M.2 slot or slots. Around 7,000MBps is the real-world ceiling for these drives, and that only in high-performance ones. The only people who might actually notice (or even be able to hit) that kind of sky-high throughput on a sustained basis are those transferring enormous files between two PCIe 4.0 M.2 drives installed on the same motherboard. (Otherwise, the source or destination drive will be a bottleneck.)

PCI Express 5.0 is the latest and by far the fastest. It offers substantial throughput increases, with maximum read and write speeds of up to 14,000MBps, effectively double those of the fastest PCIe 4.0 drives. Only the latest high-end desktops support this bus off the shelf, so you may have to build your own PC from scratch or perform a motherboard and CPU transplant on an existing desktop. Intel users will need a 12th Gen or later Core CPU with a motherboard based on Intel's Z690, Z790, or a later chipset. AMD fans must have a Ryzen 7000 or 9000 series processor on an AM5 motherboard with an X670, X670E, B650E, or later chipset. Note: The board must specifically have a PCIe 5.0-capable M.2 slot, too; not every board with chipset-level support does! (Also know: Very few laptops can leverage the peak speeds of PCIe 5.0 drives, yet.)

Third-party reviews like PCMag's, not vendor numbers, are the only substantial measures of SSD speed. In our testing of PCIe 4.0 drives (specifically via deep dives through the supporting data inside PCMark 10), we found the sky-high sequential numbers advertised by PCIe 4.0 drive manufacturers often don't have as proportional an effect on how a drive will perform when tasked with handling real-world scenarios like booting into Windows, launching games like Overwatch, or launching programs like Adobe Photoshop and Adobe Premiere. This is also true of the PCIe 5.0 SSDs we have tested.

In those tests, drives of every bus type, from PCIe 5.0 down to SATA 3.0, often can trade blows, and the best among them can take top marks away from drives that are much more expensive per gigabyte. If you're trying to get the most gaming, application, or operating system performance for the lowest cost per gig, you'll even find SATA-based options out there that remain competitive enough for most uses.

That said, if you have an M.2 slot and are shopping in capacities of 2TB or below, the price per gig starts to even out between most SATA options and M.2 PCI Express. The PCIe drives are still faster in sequential read/write operations by a lot, which is important for moving large amounts of stuff around (backing up your PC each day, for example), and if you can find one that matches the price of a competing SATA option, the M.2 PCIe should take the front of the line. Often, though, especially if updating a laptop, you'll have only one choice of drive form factor and interface.