Can I put an NVMe SSD in any M.2 slot?

No. An M.2 slot can be wired for SATA, for PCIe (NVMe), or for both. The drive may fit physically and still not be detected if the slot only carries SATA. Check the motherboard or laptop manual for which protocols each M.2 slot supports before buying.

Will a PCIe 4.0 NVMe SSD work in a PCIe 3.0 slot?

Yes. PCIe drives are backward compatible and run at the slower generation's link rate. Western Digital, for example, lists its PCIe Gen4 SN770 as compatible with PCIe Gen3 x4, Gen3 x2 and older links, at correspondingly lower peak speed.

Is a SATA SSD still much faster than a hard drive?

Yes, mainly in access time rather than peak transfer. A SATA SSD has no moving parts, so random reads that make a hard drive seek are far quicker. On sustained sequential transfers the gap is smaller: a large 7,200 RPM hard drive can sustain under 300 MB/s, while a SATA SSD is capped by the 600 MB/s interface.

What happens when an SSD reaches its TBW rating?

TBW is a warranty and endurance figure, not a hard shutoff. Western Digital's warranty, for example, ends at 5 years or the TBW limit, whichever comes first. NVMe drives report a Percentage Used value that reaches 100 when the rated endurance has been consumed; the specification says this does not necessarily mean the drive has failed.

SSD vs HDD vs NVMe: What the Difference Actually Is

An HDD stores data on spinning magnetic platters; an SSD stores it on flash memory with no moving parts. "NVMe" is not a third kind of drive: it is the protocol an SSD uses when it connects over PCIe instead of SATA, which lifts the speed ceiling from 600 MB/s (SATA 6 Gb/s) to several thousand MB/s.

So the real choice is between three combinations: a hard drive on SATA, an SSD on SATA, and an SSD on PCIe using NVMe. The connector you plug the drive into, and the shape of the drive, are separate questions again.

The three options side by side

	HDD (SATA)	SATA SSD	NVMe SSD (PCIe)
Storage medium	Spinning magnetic platters	NAND flash	NAND flash
Interface ceiling	SATA 6 Gb/s (600 MB/s)	SATA 6 Gb/s (600 MB/s)	PCIe link: Gen3 x4 = 32 GT/s raw, Gen4 x4 = 64 GT/s raw
Example real drive	Seagate IronWolf Pro 32 TB: 285 MB/s max sustained	Capped near 600 MB/s by the interface	WD_BLACK SN770 2 TB: up to 5,150 MB/s sequential read
Common form factors	3.5" (desktop), 2.5" (laptop)	2.5", M.2	M.2 (most often 2280)
Moving parts	Yes	No	No

The drive figures come from the manufacturers' data sheets listed in Sources. They are sequential (large-file) numbers, the best case for every drive type.

Why SATA caps an SSD at about 600 MB/s

SATA's current revision runs at 6 gigabits per second. SATA-IO, the body that maintains the standard, says the interface "transmits information at 600MB/s, however not all 600MB/s are realized as the user data payload" because commands and handshakes share the link. That 600 MB/s is a ceiling for the cable, not the flash. Fast flash chips can deliver more, so on SATA the interface becomes the bottleneck.

A naming note: you will see "SATA III" on shop listings. SATA-IO's naming guidelines ask manufacturers not to use "SATA III" or "SATA 3.0" and to call it "SATA 6Gb/s" instead. They mean the same interface.

How fast PCIe is for an NVMe drive

NVMe drives talk to the CPU over PCI Express lanes. Intel gives the per-lane transfer rates as 8 GT/s for PCIe 3.0, 16 GT/s for PCIe 4.0 and 32 GT/s for PCIe 5.0, and notes that these are theoretical figures before encoding overhead. Each generation doubles the previous one. Intel also notes that modern M.2 PCIe SSDs use four lanes (x4).

Multiply it out and an x4 drive has a raw link of 32 GT/s on PCIe 3.0, 64 GT/s on 4.0 and 128 GT/s on 5.0. Those are raw rates, and usable throughput is lower after encoding. The practical evidence is in the data sheets: Western Digital lists the SN770's interface as "PCIe Gen4 16GT/s, up to 4" lanes and rates the 2 TB model at 5,150 MB/s, more than eight times the SATA ceiling.

How much of that you notice depends on the work. Copying a 50 GB video file is limited by sequential throughput, where NVMe shines. Booting, opening apps and loading small files depend more on access latency, where any SSD is a huge step up from a hard drive and the SATA-versus-NVMe gap is smaller.

M.2 is a shape, not a speed (the most common mistake)

M.2 describes the physical card. The number after it gives the size in millimetres: an M.2 2280 drive is 22 mm wide and 80 mm long, which matches the 22 × 80 mm dimensions in the SN770 data sheet. An M.2 card can carry either of two very different interfaces:

M.2 SATA: same protocol and the same 600 MB/s ceiling as a 2.5" SATA SSD, just in a smaller package.
M.2 NVMe: PCIe lanes, usually x4, with the much higher ceiling described above.

The two look almost identical. Before buying, check two things:

The drive's spec sheet: it will say "SATA" or "PCIe Gen3/Gen4/Gen5 x4, NVMe".
Your motherboard or laptop manual: each M.2 slot lists which protocols it supports. Some slots take only SATA, some only PCIe, some both. A drive in a slot that doesn't carry its protocol will not be detected.

2.5" drives are simpler: a 2.5" SATA SSD uses the same data and power connectors as a 2.5" laptop hard drive, so it drops into the same bay.

How long an SSD lasts: TBW

Flash cells wear out after a finite number of writes, so SSD makers publish an endurance rating in TBW (terabytes written). Western Digital rates the 2 TB SN770 at 1,200 TBW and notes the figure is "calculated using JEDEC client workload (JESD219)". Its warranty is 5 years "or Max Endurance (TBW) limit, whichever occurs first."

To put 1,200 TBW in perspective with your own numbers: writing 50 GB every day is 18.25 TB a year, so it would take about 66 years to reach 1,200 TB. Heavy video or database work writes far more, so do the same sum with your real daily volume.

You don't need to guess your writes. NVMe drives keep a running total in their SMART log ("Data Units Written") and an estimate called "Percentage Used". The NVMe specification defines Percentage Used as a "vendor specific estimate of the percentage of NVM subsystem life used", where 100 means the rated endurance has been consumed but "may not indicate an NVM subsystem failure." The SSD health guide shows how to read both.

Hard drives are rated differently, as a workload limit per year: Seagate rates IronWolf Pro drives at 550 TB/year with a 5-year limited warranty.

When a hard drive still makes sense

Bulk storage where capacity matters most. Seagate's IronWolf Pro line goes up to 32 TB in a single 3.5" drive. Backups, media libraries and NAS arrays read and write large files sequentially, which is the access pattern hard drives handle best.
Cold archives. Data you write once and rarely read gains nothing from SSD access times.
Cost per terabyte. Divide the current price by the capacity for each option you are considering, at the size you actually need. Prices move, so do the sum on the day you buy rather than trusting an old rule of thumb.

Put the operating system and applications on an SSD in every case. Booting and launching programs are made of many small random reads, which is exactly where a hard drive's seek time hurts and an SSD has no equivalent delay.

Which one to buy

Laptop or desktop boot drive, board has an M.2 PCIe slot: NVMe SSD. Compare its price with a SATA SSD of the same capacity; if the difference is small, NVMe gives you the higher ceiling for free.
Older machine with only SATA ports or a 2.5" bay: 2.5" SATA SSD. You will still get the large access-time improvement over a hard drive.
Second drive for large files, backups or a NAS: hard drive, sized for growth.
PCIe 5.0 drive in a PCIe 4.0 or 3.0 slot: it works, at the slot's speed. Pay for the faster generation only if your board supports it.

Frequently asked questions

Can I put an NVMe SSD in any M.2 slot?: No. An M.2 slot can be wired for SATA, for PCIe (NVMe), or for both. The drive may fit physically and still not be detected if the slot only carries SATA. Check the motherboard or laptop manual for which protocols each M.2 slot supports before buying.
Will a PCIe 4.0 NVMe SSD work in a PCIe 3.0 slot?: Yes. PCIe drives are backward compatible and run at the slower generation's link rate. Western Digital, for example, lists its PCIe Gen4 SN770 as compatible with PCIe Gen3 x4, Gen3 x2 and older links, at correspondingly lower peak speed.
Is a SATA SSD still much faster than a hard drive?: Yes, mainly in access time rather than peak transfer. A SATA SSD has no moving parts, so random reads that make a hard drive seek are far quicker. On sustained sequential transfers the gap is smaller: a large 7,200 RPM hard drive can sustain under 300 MB/s, while a SATA SSD is capped by the 600 MB/s interface.
What happens when an SSD reaches its TBW rating?: TBW is a warranty and endurance figure, not a hard shutoff. Western Digital's warranty, for example, ends at 5 years or the TBW limit, whichever comes first. NVMe drives report a Percentage Used value that reaches 100 when the rated endurance has been consumed; the specification says this does not necessarily mean the drive has failed.