The M.2 Expansion Slot is a small, hidden PCIe slot that can be used to add an extra 2.5-inch drive to your computer. This is great for people who want to add more storage space to their computer, or for people who want to use a different type of drive in their computer. The M.2 Expansion Slot is also great for people who want to use a different type of motherboard, or for people who want to use a different type of graphics card.

There’s a new open format sweeping the world of performance PCs, and it’s…well, complicated. The M.2 format is designed for manufacturers to replace a variety of specific devices, do it in a tiny space, and require very little power. But actually upgrading to an M.2 drive or accessory requires a little forethought.

Where Did M.2 Come From?

Formerly known as Next Generation Form Factor (NGFF), the M.2 format is technically a replacement for the mSATA standard,  which was popular with manufacturers of super-compact laptops and other small gadgets. That may seem surprising, since most M.2 drives sold at retail are intended for use in full-sized desktops, but M.2 has effectively replaced mSATA hard drives and SSDs in compact laptops like Apple’s MacBook or Dell’s XPS 13. They’re simply sealed within the bodies and unable to be upgraded by most users.

What Can It Do?

M.2 is more than just an evolutionary form factor. Potentially, it could supersede the whole aging Serial ATA format altogether. M.2 is a slot that can interface with SATA 3.0 (the cable that’s probably connected to your desktop PC’s storage drive right now), PCI Express 3.0 (the default interface for graphics cards and other major expansion devices), and even USB 3.0.

That means that—potentially—any storage or disk drive, GPU or port expansion, or low-power gadget that uses a USB connection, could all be mounted on a card plugged into the M.2 slot at the same time. The reality is a little more complicated—for example, a single M.2 slot only has four PCI Express lanes, a quarter of the total generally desired for graphics cards—but the flexibility for this tiny little slot is impressive.

When using the PCI bus instead of the SATA bus, M.2 devices can transfer data at anywhere from 50% to approximately 650% faster than standard SATA, depending on the capabilities of the motherboard and the M.2 card itself. If you have the opportunity to use an M.2 SSD on a motherboard that supports PCI generation 3, it can be significantly faster than a regular SATA drive.

What Devices Use The M.2 Slot?

At the moment, M.2 is primarily used as an interface for super-fast SSDs, both on laptops and desktops. If you walk into a computer hardware store and ask for an M.2 drive—assuming you can find a retail computer store still in operation, of course—they’ll almost certainly show you an SSD with an M.2 connector.

Some laptop designs also use an M.2 port as their means of wireless connection, mounting tiny, low-powered cards that combine Wi-Fi and Bluetooth radios. This is less common for desktops, where the ease of a USB dongle or PCIe 1x card is preferred (though there’s no reason you couldn’t do it on a compatible motherboard).

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Finally, some companies are starting to expand the use of the slot into categories that don’t broadly fit storage or expansion. While no one’s made an M.2 graphics card yet, Intel is selling its speed-boosting cache storage, “Optane,” in M.2 format for consumers.

Does My Computer Have an M.2 Slot?

If your PC was made or assembled in the last few years, it probably has an M.2 slot. Unfortunately, the flexibility of the format means that actually using it isn’t as simple as just plugging in a card.

M.2 cards come with two major compatibility variables: length and key. The first is fairly obvious—your computer needs to have enough physical space to support the length of the card you want to use. The second variable—how the card is keyed—just means the card connector must match the slot you’ll be plugging it into.

M.2 Length

For desktops, length isn’t typically a problem. Even a tiny Mini-ITX motherboard can easily make room for the maximum length M.2 PCB, which is 110 millimeters long. Some cards are as short as 30mm. You generally want a card to be the size intended for use by your motherboard manufacturer, as an indentation on the end of the PCB allows for a small screw to hold it securely in place.

All M.2 drives use the same width determined by the connection. The “size” is expressed in the following format; check for compatibility with your laptop or motherboard when picking one out:

M. 2 2230: 22 millimeters wide by 30 millimeters long. M. 2 2242: 22 millimeters wide by 42 millimeters long. M. 2 2260: 22 millimeters wide by 60 millimeters long. M. 2 2280: 22 millimeters wide by 80 millimeters long. M. 2 2210: 22 millimeters wide by 110 millimeters long.

Some motherboards are flexible, offering mounting holes for the retention screw at some or all of these intervals.

M.2 Key

While the M.2 standard uses the same 22 millimeter-wide slot for all cards, it’s not necessarily the exact same slot. Since M.2 is designed to be used with so many different kinds of devices, it has some frustratingly similar-looking ports.

B Key: uses a gap in the right side of the card (left side of the host controller), with six pins to the right of the gap. This configuration supports PCIe x2 bus connections. M Key: uses a gap in the left side of the card (right side of the host controller), with five pins to the left of the gap. This configuration supports PCIe x4 bus connections for twice the data throughput. B+M Key: uses both of the above gaps, with five pins on the left side of the card and six on the right. Because of the physical design, B+M Key cards are limited to PCIe x2 speeds.

M.2 cards with a B Key interface can only fit into a B Key host slot, and likewise for M Key. But cards with a B+M Key design can fit in either a B or an M host slot, since they have gaps for both.

Check your laptop or motherboard specification to see which one is supported. We recommend seeing the documentation instead of “eyeballing” the slot, since the two key standards can be easily confused.

What Do I Need to Install an M.2 Card?

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Not much. Most M.2 cards are SSDs and are automatically recognized by your operating system based on AHCI drivers. For Windows 10, most Wi-Fi and Bluetooth cards are automatically recognized as well, with generic drivers activated immediately or specific drivers downloaded later. However, you may need to enable the M.2 slot via a setting in your computer’s BIOS or UEFI. You’ll also want a screwdriver to put in the retention screw.

Can I Add An M.2 Card if My PC Doesn’t Have a Slot?

For laptops, the answer is no—the design of modern laptops is so compact that there’s no space for any kind of non-planned expansion. If you use a desktop, you’re in luck. There are plenty of adapters for sale that use the PCIe x4 slot already on your motherboard. However, if your motherboard can’t boot from PCIe, then you won’t be able to set that M.2 drive as your boot drive, which means you won’t benefit from a lot of the speed. So keep that in mind—if you want the full benefits of an M.2 drive, you’ll probably need a motherboard that supports it.

Image source: iFixIt, Intel, Kingston