Storage interface speeds at a glance
Storage drives connect over different buses, and each bus has a hard speed ceiling. This reference lists the common interfaces — from SATA hard-drive links to the latest PCIe 5.0 NVMe — with the maximum sequential transfer speed you can expect, plus the connector and a short note for each.
How it works
Every serial bus advertises a raw bit rate (for example SATA III at 6.0 Gbit/s), but the usable data rate is lower because of line encoding. SATA uses 8b/10b encoding, so every 10 transmitted bits carry 8 bits of data — a 20% tax that turns 6.0 Gbit/s into roughly 600 MB/s. PCIe 3.0 and newer use the far more efficient 128b/130b encoding, leaving only about 1.5% overhead, which is why PCIe scales so much better.
For PCIe, total bandwidth is per-lane rate × number of lanes. NVMe SSDs typically use an x4 link, so a PCIe 4.0 x4 drive gets four lanes of roughly 1.97 GB/s each, about 7.88 GB/s. Each PCIe generation doubles the per-lane rate, so the same x4 slot moves from ~3.94 GB/s (Gen3) to ~7.88 GB/s (Gen4) to ~15.76 GB/s (Gen5).
Quick comparison across major interfaces
| Interface | Max seq. speed | Notes |
|---|---|---|
| SATA I | ~150 MB/s | First generation, rarely seen now |
| SATA II | ~300 MB/s | Common on older HDDs |
| SATA III | ~600 MB/s | Current SATA ceiling; most HDDs and some SSDs |
| SAS 12G | ~1.2 GB/s | Enterprise drives, dual-port capable |
| NVMe PCIe 3.0 x4 | ~3.5 GB/s | First-gen NVMe consumer SSDs |
| NVMe PCIe 4.0 x4 | ~7 GB/s | Current mainstream NVMe tier |
| NVMe PCIe 5.0 x4 | ~12–14 GB/s | Latest consumer Gen5 drives |
| USB 3.2 Gen 2x2 | ~2.4 GB/s | External drive ceiling on this spec |
| Thunderbolt 3/4 | ~2.75 GB/s | External enclosure ceiling |
The M.2 connector confusion
M.2 is physically a small slot — about the size of a stick of gum — that supports both SATA and NVMe protocols depending on the drive installed. You cannot tell from the slot itself which protocol is in use. Check:
- The drive’s label: NVMe drives say NVMe; SATA M.2 drives often say SATA or NGFF.
- The key notch: M-key slots (right side notch only) support both SATA and NVMe. B+M-key drives (notches on both sides) are usually SATA.
- Your motherboard spec sheet: some M.2 slots support only SATA or only NVMe.
An M.2 NVMe drive in a slot wired for SATA will either not work or run at SATA speeds, depending on the motherboard.
Sequential speed versus random IOPS
The speeds in this reference are sequential — reading or writing a single large file in one continuous stream. Real-world responsiveness often depends more on random 4K IOPS, which measures how quickly a drive handles many small unrelated reads and writes simultaneously. Operating system loads, database queries, and application launches are almost entirely random workloads.
NVMe drives pull far ahead of SATA on random IOPS because NVMe was designed for solid-state flash and eliminates the overhead of the old AHCI command queue layer. Even a mid-range PCIe 3.0 NVMe drive typically delivers several times the random IOPS of the fastest SATA SSD, which is why NVMe drives feel snappier for everyday use even when sequential speed numbers look similar.
Tips and notes
- M.2 is a slot shape, not a protocol — always check whether an M.2 drive is SATA or NVMe before assuming it is fast.
- The B-key, M-key, and B+M-key notches on M.2 modules determine which slots accept them; NVMe drives are M-key.
- External enclosures inherit the host port’s ceiling. A Gen4 NVMe stick in a USB 3.2 Gen 2 (10 Gbit/s) enclosure will run at roughly 1 GB/s, not 7 GB/s.
- Sequential speed is only one dimension; random 4K IOPS often matters more for OS responsiveness and is not captured by these bus ceilings.
- Gen5 NVMe drives run hot under sustained load; check thermal throttling behavior if planning one for a compact form factor without airflow.