"I need an M.2 SSD" is something I hear often, but on its own it isn't enough to buy the right drive. M.2 describes the physical shape of the connector, not the way the drive talks to the computer. Two M.2 SSDs that look identical can be one very fast and the other much slower, and one of them might not even work in your computer.
Three concepts to keep apart
To find your way you need to distinguish three levels:
- Form factor: the physical shape of the drive. Examples: 2.5-inch drive, M.2 card, PCIe add-in card.
- Interface: the electrical connection. It can be SATA or PCI Express.
- Protocol: the language used to communicate. AHCI was born for SATA drives; NVMe was designed for flash memory over PCI Express.
An "M.2 NVMe" SSD is therefore a drive in M.2 format that uses the PCIe bus with the NVMe protocol. An "M.2 SATA" SSD looks the same, but internally it speaks SATA like an old 2.5-inch drive.
SATA: simple and universal
The SATA interface in its third revision has a theoretical limit of 6 Gbit/s, which in practice translates to roughly 550 MB/s in sequential reads. All good-quality SATA SSDs get more or less there: what changes between models is endurance, cache management and reliability, not peak speed.
The 2.5-inch SATA format is still very useful for upgrading older computers or adding space to a desktop that still has free SATA ports.
NVMe and PCIe generations
An NVMe SSD normally uses four PCIe lanes. Maximum speed depends on the generation:
| PCIe generation | Indicative sequential speed (x4) |
|---|---|
| 3.0 | up to about 3.5 GB/s |
| 4.0 | up to about 7 GB/s |
| 5.0 | over 10 GB/s in high-end models |
Generations are backward compatible: a PCIe 4.0 SSD works in a PCIe 3.0 slot, but runs at the slot's speed. The reverse also holds.
To be honest, in everyday use the difference between a good PCIe 3.0 SSD and a PCIe 5.0 one is barely noticeable: booting and opening programs depend mostly on random access and latency, not sequential speed. Newer generations shine when copying huge files and in some professional workflows, and they often run hotter.
M.2 connector keys
The M.2 connector has notches, called keys, that indicate which signals it supports:
- M key: typical of NVMe SSDs, supports up to four PCIe lanes;
- B key: used by old SATA SSDs and some mobile network modules;
- B+M key: two notches, typical of M.2 SATA SSDs, which can therefore physically fit more slots.
The crucial point is that fitting the slot doesn't mean working. An M.2 slot may support only NVMe, only SATA, or both. The only way to know is to check the motherboard or laptop manual.
Sizes: what 2280 means
The number after "M.2" gives width and length in millimetres. A 2280 SSD is 22 mm wide and 80 mm long, and is by far the most common. There are also 2230 and 2242, shorter, used in compact laptops and handheld consoles, and 22110, longer, used in some servers and workstations. Before buying, check which lengths the slot accepts.
Lane sharing: the manual's trap
On many motherboards, M.2 slots share resources with other components. Fitting an SSD in a certain slot may disable two SATA ports or reduce the lanes of a PCIe slot. The manual usually shows this in a table: reading it beforehand saves you from thinking a drive or port is faulty.
Heat and heatsinks
Fast NVMe SSDs can get hot during sustained writes and, past a certain temperature, slow down to protect themselves. Many motherboards have a heatsink for the M.2 slot: remove the protective film from the thermal pad before refitting it. Laptops have little room, so it's better to choose less extreme models.
How to choose
- Check in the manual the slot type, supported protocol and accepted lengths.
- Choose NVMe if the slot supports it: even a PCIe 3.0 model is much faster than SATA.
- For an old computer without M.2, a 2.5-inch SATA SSD is still a huge upgrade over a hard disk.
- Weigh endurance (TBW), warranty and DRAM cache more than peak speed.
In short
- M.2 is a form factor; SATA and NVMe are different ways of communicating.
- An M.2 slot may accept only NVMe, only SATA or both: the manual tells you.
- PCIe generations are backward compatible and run at the lower speed.
- 2280 is the most common size, but some devices need shorter drives.
- In everyday use, a good NVMe drive of any generation is already very fast.
