M.2 SSDs with B+M keys are one of the more confusing parts of PC hardware. They look identical physically, but can behave completely differently depending on the protocol.

What is a B+M Key SSD?

A B+M key SSD has two notches in the connector: This allows the drive to physically fit into both:
  • B-key slots
  • M-key slots
Convenient, right? However, this does NOT determine whether the drive is NVMe or SATA.

Identical Appearance, Different Protocols

Two drives can look identical:
  • B+M key
  • Same size (e.g. 2280 or 2230)
But they can use completely different protocols:
  • NVMe (PCIe)
  • SATA
And they are NOT interchangeable.

NVMe vs SATA (What Actually Changes)

Type Keying Protocol Interface Typical Speed Notes
M-Key NVMe M PCIe (NVMe) PCIe x4 Fast (2000–7000+ MB/s depending on gen) Standard for modern NVMe SSDs
B+M Key NVMe B+M PCIe (NVMe) PCIe x2 (usually) Moderate (1000–2000 MB/s) Less common, used for compatibility
B+M Key SATA B+M SATA SATA III ~500–600 MB/s Very common in older/lower-cost SSDs
B-Key (SATA/PCIe) B SATA or PCIe PCIe x2 or SATA Varies Rare for SSDs, more common in WWAN/other modules

Important Note About B+M Key Drives

B+M key SSDs are most commonly SATA, but they can also be NVMe. This means that if you have a B+M drive there's a chance that your drive is SATA and not PCIe. Because of this, you cannot determine the protocol from the connector alone. Always verify the drive model before buying an enclosure or adapter. M-Key SSDs are effectively always NVMe in consumer hardware. B+M Key SSDs require verification, as they can be either SATA or NVMe.

Why They Are Not Compatible

It is a common misconception that the only difference is speed, and that using the wrong enclosure will just result in slower performance. Reason:
  • NVMe uses PCIe signaling
  • SATA uses SATA signaling
So it would not just be slower, it would be electrically incompatible, and drive would not be detected at all.

Enclosure Compatibility

Drive Type NVMe Enclosure SATA Enclosure
B+M Key NVMe Works Does NOT work
B+M Key SATA Does NOT work Works

Why B+M Exists

B+M key SSDs were designed for maximum physical compatibility. This allows manufacturers to create drives that:
  • Fit more devices
  • Work in both B-key and M-key slots
However, this flexibility comes at the cost of confusion.

How to Tell If Your B+M SSD is NVMe or SATA

The notch layout alone is not enough. Use this method:
  1. Check the model number (best method)
  2. Look for "NVMe" or "PCIe" on the label
  3. Check manufacturer specifications
Do NOT rely on:
  • Number of notches
  • Connector shape

Quick Summary

B+M Key = physical compatibility only

B+M NVMe → uses PCIe → works in NVMe enclosure only
B+M SATA → uses SATA → works in SATA enclosure only

No fallback. No cross-compatibility.

Conclusion

B+M key SSDs are physically flexible but electrically strict. Always match the protocol (NVMe vs SATA), not just the connector. If you get that right, everything else becomes simple. B+M key SSDs can be either SATA or NVMe. SATA is slower, but the key difference is compatibility - SATA and NVMe are not interchangeable.