An M.2 drive with a B key notch is typically SATA-based, sometimes PCIe x2, and won’t fit an M-key-only slot.
For the full breakdown, see our best B Key SSD guide.
M.2 drives use notch patterns called keys so a card sits only in a socket built for it. B-key is not a separate SSD standard—it is one of several M.2 card key types, and Dell’s M.2 documentation lists B-key among them and associates it with SSD use.
What Does A B Key SSD Fit?
A B key SSD fits M.2 sockets wired for B-key cards, and a dual-notch B+M drive fits both B-key and M-key sockets physically. Physical fit is not function: a SATA M.2 drive needs SATA support from the host slot, and an NVMe drive needs PCIe/NVMe support from the slot and firmware. If a system only has an M-key-only socket, a B-key-only SSD will not physically fit.
Dell’s support page on M.2 cards says SSDs using the SATA bus tend to use B-key connectors, while PCIe x2 drives may use B or M keys for compatibility. That is why B+M drives are so common: they give a SATA SSD the best chance of fitting a motherboard, whichever keying that board’s slot uses. MSI’s FAQ makes the same pairing, describing a B key edge connector as working with its Socket 2 B-key slots, which are SATA-oriented.
Protocol support is where buyers get tripped up. A B+M drive may slide into an M-key socket, but if the socket is wired for NVMe only and the drive is SATA, the system may not recognize it at all. The physical notch is about connector compatibility; the interface is about the host controller.
B Key Vs M Key: The Difference That Matters
The measurable difference comes down to interface and lane count. That is why M-key NVMe drives are the standard path for high-speed builds, while B-key and B+M drives show up in SATA-based systems, moderate-bandwidth deployments, and embedded hardware.
B-key-only modules are rarer for consumer SSDs; you are more likely to see them in non-SSD M.2 modules or specific embedded uses. Many consumer SATA M.2 drives ship as B+M drives so they can fit a wider range of slots, but the drive still runs at SATA speed no matter how many notches it carries.
| Feature | B Key | M Key |
|---|---|---|
| Edge connector notch | Pins 12–19 | Opposite edge |
| Typical interface | SATA, sometimes PCIe x2 | PCIe, commonly NVMe |
| PCIe lanes per ASUS | Up to 2x | Up to 4x |
| Fits B-key socket | Yes | No |
| Fits M-key socket | No | Yes |
| Dual-notch B+M variant | Fits both sockets | Fits both sockets |
| Common buyer mistake | Assumed to be NVMe | Assumed to be SATA |
A quick read of the table covers most of the confusion, but always check the actual motherboard manual for one detail: whether the specific M.2 slot is SATA, PCIe, or both. A board can have one M.2 slot that supports NVMe and another that only supports SATA, and the keying alone won’t tell you which is which. A B-key slot may not support NVMe even if another M.2 slot on the same board does.
If you’re ready to buy rather than research compatibility one more time, our tested list of the best B-key SSDs narrows down the drives that work in B-key sockets.
What To Check Before You Buy
Read the drive’s label or model number for SATA versus NVMe before ordering; the notch shape is not enough. Then check the motherboard, laptop, or manual for the exact M.2 socket keying and the interfaces that socket supports. For the final word, use the system manufacturer’s support page rather than a forum post, because slot wiring varies by model and revision.
- Not every M.2 SSD is NVMe. Many B-key and B+M drives are SATA and will bottleneck if you expect PCIe x4 speed.
- A dual-notch B+M card is about compatibility, not speed. It still runs on whatever interface the host provides.
- Size matters for physical fit (2230, 2242, 2260, 2280), but keying and protocol decide whether the drive works at all.
- Never force an M.2 card into a slot. The keying is designed to prevent damage; if it doesn’t seat, it doesn’t belong there.
- For a modern NVMe performance build, a B-key SSD is generally the wrong choice. Plan on an M-key PCIe x4 drive instead.
Match the notch, match the interface, and the drive will behave exactly as labeled.
FAQs
Can I Put A B+M SSD In An M-Key Slot?
A SATA B+M drive in an NVMe-only M-key slot usually isn’t recognized, since SATA is not wired on that socket. Check the motherboard manual first.
Is A B-Key SSD Slower Than An M-Key SSD?
Peak throughput is usually lower for B-key drives because they commonly run SATA or PCIe x2, while M-key NVMe drives use PCIe x4. That doesn’t make B-key drives useless: for SATA-based systems and everyday workloads, they work fine and cost less.
Why Do Some M.2 Drives Have Two Notches?
Two notches mean a B+M keyed drive, designed for physical compatibility with both B-key and M-key sockets. The drive still needs the host to provide the matching interface, and the dual notch does not make it faster than a single-keyed drive.
References & Sources
- Dell. “How to Distinguish the Differences Between M.2 Cards.” Explains B-key SATA use and B/M key compatibility for PCIe x2.