M.2 SSD Explained: Keys, NVMe vs SATA and Sizes for the A+ Exam

M.2 is a form factor, not a protocol. That single sentence resolves most of the confusion around these drives, and it is what the A+ exam is really testing.

An M.2 slot is a physical connector on the motherboard. What runs across it may be SATA or NVMe, and the performance difference between the two is large.

NVMe versus SATA over M.2

	M.2 SATA	M.2 NVMe
Protocol	SATA (AHCI)	NVMe over PCIe
Speed ceiling	about 600 MB/s	3,500 MB/s and far beyond
Typical keying	B+M (two notches)	M only (one notch)
Queue depth	1 queue, 32 commands	65,535 queues

An M.2 SATA drive is capped at the same 600 MB/s as any other SATA III device, because it is using the SATA protocol — the connector changed, the bus did not. NVMe talks directly over PCIe lanes and skips the translation layer that AHCI imposes, which is why it is several times faster.

The deep queue matters more than the headline sequential number. AHCI was designed for spinning disks with one head; NVMe was designed for flash that can serve thousands of requests in parallel. Under a multitasking load the gap is wider than the benchmark figures suggest.

Keys

The notch position in the edge connector is the key, and it determines what will physically fit.

B key — notch nearer the left, supports SATA and PCIe up to x2.
M key — notch nearer the right, supports PCIe up to x4. NVMe drives use this.
B+M key — two notches, fits either a B or an M slot. Usually a SATA drive.

The practical inference for the exam: two notches usually means SATA, one notch usually means NVMe. A B+M drive fits an M-key slot, but a single-notch M-key NVMe drive will not fit a B-key slot.

Fitting is not the same as working, which brings us to the failure everyone meets.

Why a drive fits but is not detected

The most common M.2 support scenario, and worth knowing in order.

The slot supports only one protocol. Many M.2 slots are NVMe-only or SATA-only. A drive of the wrong type seats perfectly and never appears. Check the motherboard manual — this is the first thing to verify.

Lane sharing. On many boards, populating a particular M.2 slot disables SATA ports, or the second M.2 slot runs at reduced lanes. A drive that appeared before may vanish after another is installed.

The drive is not initialised. It shows in firmware and in Disk Management but has no partition or volume.

Firmware settings. The slot may be disabled, or set to the wrong mode.

Not fully seated or the standoff screw is missing. M.2 drives sit at an angle until screwed down.

Sizes

The size code is width and length in millimetres. 2280 means 22 mm wide, 80 mm long, and is the desktop and most-laptop standard. You will also meet 2230, 2242, 2260 and 22110. Width is almost always 22 mm; the length is what varies, and the board must have a standoff at the matching position.

Compact laptops and handhelds often take only 2230, which limits the upgrade options.

PCIe generations

NVMe performance depends on the PCIe generation and the number of lanes. Gen 3 x4 gives roughly 3,500 MB/s, Gen 4 x4 roughly 7,000, Gen 5 x4 roughly 14,000.

Generations are backward compatible, negotiating down to the lower of drive and slot. A Gen 4 drive in a Gen 3 slot works at Gen 3 speed — the same negotiate-down behaviour that catches people with SATA revisions, and a likely exam scenario.

Thermals

Fast NVMe drives generate real heat and will thermally throttle, dropping performance sharply during sustained transfers. The symptom is distinctive: excellent speed for the first several seconds of a large copy, then a sudden fall. Heatsinks and adequate airflow are the fix, and many boards now ship with M.2 heatsinks for this reason.

Related form factors

mSATA is the older, larger predecessor, SATA-only and now obsolete. U.2 is an enterprise connector carrying NVMe to a 2.5-inch drive, usually hot-swappable. M.2 slots also host Wi-Fi and Bluetooth cards, which use A or E keys — so not every M.2 slot on a board is a storage slot.

What to remember

M.2 is a form factor; the protocol may be SATA or NVMe. One notch (M key) usually means NVMe, two notches (B+M) usually means SATA. A slot may support only one protocol, so a drive can fit and still not appear. 2280 is 22 mm by 80 mm. PCIe generations negotiate down. Sustained-write slowdowns are thermal throttling.

Related reading: SATA connectors and speeds · eSATA · SODIMM memory