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USB speed names decoded for external SSDs: USB 3.2 Gen 2, USB4 and Thunderbolt

The same USB-C plug can carry 480 Mbps or 40 Gbps. Learn the names, the real speed behind each and how to check your port and cable.

By The PC Journal9 min read

The short answer

The speed of an external SSD is set by the slowest of three things: the drive, the port and the cable. USB 3.2 Gen 1 is 5 Gbps, Gen 2 is 10 Gbps, Gen 2x2 is 20 Gbps, USB4 is 20 or 40 Gbps and Thunderbolt 3 and 4 are 40 Gbps, so a USB-C plug alone tells you nothing about speed.

The names, in plain terms

USB naming was relabelled several times. What was once called USB 3.0 is now USB 3.2 Gen 1 and runs at 5 Gbps. What was USB 3.1 Gen 2 is now USB 3.2 Gen 2 at 10 Gbps. USB 3.2 Gen 2x2 uses two lanes to reach 20 Gbps, but it is less common on PCs and needs both the port and the drive to support it. USB 2.0 is still found on some ports at 480 Mbps.

USB4 is built on the Thunderbolt 3 protocol and runs at 20 Gbps or 40 Gbps depending on the port. Thunderbolt 3 and Thunderbolt 4 are 40 Gbps, with Thunderbolt 4 setting a higher minimum set of features. A newer generation of USB4 and Thunderbolt reaches 80 Gbps, but those ports are only on recent hardware and the matching SSDs are rare.

Gbps are bits, not bytes. Divide by eight to estimate megabytes per second and then subtract overhead. A 5 Gbps link tops out near 500 MB/s in practice, a 10 Gbps link near 1,000 MB/s and a 20 or 40 Gbps link is faster still, if the SSD can use it.

Quick reference

Figures are the link speeds in the standards. Real-world file copies are lower because of protocol overhead, the SSD controller and the files themselves.

NameLink speedPractical ceilingWhat it suits
USB 2.0480 MbpsAbout 40 MB/sMice, keyboards; too slow for an SSD
USB 3.2 Gen 1 (old USB 3.0)5 GbpsAround 400 to 450 MB/sHard drives, entry-level portable SSDs
USB 3.2 Gen 210 GbpsUp to about 1,000 MB/sMost portable NVMe SSDs
USB 3.2 Gen 2x220 GbpsUp to about 2,000 MB/sFaster NVMe SSDs, on supported ports
USB4 (20 or 40 Gbps)20 or 40 GbpsDepends on the port and SSDHigh-speed SSDs, docks
Thunderbolt 3 / 440 GbpsDepends on the SSDFast NVMe SSDs and docks

Check what you actually have

  1. Read the port label or the manual

    Look at your motherboard or laptop manual for the exact speed of each USB-C and USB-A port. A lightning-bolt symbol often means Thunderbolt, and an SS with 5, 10 or 20 marks the speed, but not every maker labels ports.

  2. Check the cable

    A charging cable can be limited to USB 2.0 speed. Use the cable that came with the SSD, or one rated for the speed you want. For 40 Gbps, look for a cable that says it supports 40 Gbps, and keep it short.

  3. Check what the drive is rated for

    The product page should give the interface speed and, if given, sequential read and write. Make sure the stated speed is on the same interface you will use.

  4. Run a real copy

    Copy a large file and watch the transfer rate in Windows. If it sits near 400 MB/s, the link is probably running at 5 Gbps, so check the port and the cable.

Compatibility notes

  • The USB-C connector is only a shape. It can carry USB 2.0, USB 3.2 at 5, 10 or 20 Gbps, USB4 and Thunderbolt, so identify the speed from the port's specification, not from the plug.
  • USB4 and Thunderbolt drives generally fall back to slower speeds on older ports, but a drive that needs a Thunderbolt-only feature may not work at all, so confirm the maker's compatibility notes.
  • SATA SSDs inside a portable enclosure are limited by SATA at roughly 550 MB/s, so a 20 Gbps port will not make them faster.

Mistakes to avoid

  • Paying for a 20 Gbps drive and using a 5 Gbps port

    Instead: Match the SSD to the fastest port you actually have, or buy the cheaper 10 Gbps model.

  • Using the phone charging cable

    Instead: Use the cable in the box, or a cable marked for the right speed.

  • Reading Gbps as MB/s

    Instead: Divide by eight, then allow for overhead.

Where to go next

  1. Best external SSDs
  2. Best NVMe portable SSDs
  3. NVMe vs SATA SSD
  4. Installing and setting up an external SSD

Sources

  1. USB Implementers ForumUSB-IF · Reference
  2. Microsoft LearnMicrosoft · Reference

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