2026年10月23日
USB 3.2 Gen 1 / Gen 2 / Gen 2×2：ストレージ仕様書の速度階層を読み解く
USB-IF の命名規則は、同じハードウェアを世代ごとに改名し続けています。本ガイドは現在の各表記を実際の転送レートに紐づけ、仕様書のどこが誤解を招きうるかを示し、USBメモリ・メモリーカード・ポータブルSSD がそれぞれどの階層を必要とするかを解説します。
この記事は英語でご覧いただけます。
Every storage buyer eventually meets the same wall: three products, three different interface labels, no obvious way to compare them. The confusion is deliberate on nobody’s part — the USB Implementers Forum renamed the same generations more than once, so a 2015 product and a 2024 product can carry different names for identical bandwidth. This guide maps the labels to the rates that actually matter when you are specifying a programme.
The naming history, compressed
The original names were simple: USB 2.0 (480 Mb/s), then USB 3.2 Gen 1 (5 Gb/s, formerly USB 3.0), USB 3.2 Gen 2 (10 Gb/s, formerly USB 3.1), and USB 3.2 Gen 2×2 (20 Gb/s). The 2019 renaming folded all of them under the USB 3.2 umbrella, which is why a current spec sheet may say USB 3.2 Gen 1 where a 2016 one said USB 3.0 — same hardware, new label. USB4 later added its own tiering (20 and 40 Gb/s) on top.
What each tier delivers in practice
| Label | Signalling rate | Realistic throughput | Typical use |
|---|---|---|---|
| USB 2.0 | 480 Mb/s | about 35–40 MB/s | Bulk documents, giveaways, low-cost promotion |
| USB 3.2 Gen 1 (ex 3.0) | 5 Gb/s | about 400–450 MB/s | Standard flash drives, card readers |
| USB 3.2 Gen 2 (ex 3.1) | 10 Gb/s | about 900–1,000 MB/s | Fast flash drives, portable SSDs |
| USB 3.2 Gen 2×2 | 20 Gb/s | about 1,800–2,000 MB/s | High-speed portable SSDs, dual-lane devices |
Throughput is always below the signalling rate: protocol overhead, the controller, and the flash itself all take a share. A drive labelled 10 Gb/s that writes at 300 MB/s is not defective — it is limited by its controller or its NAND, not by its port.
Where a spec sheet misleads
- Read speed quoted without write speed. Sequential read is the flattering number; sustained write is where controllers show their class. Ask for both.
- Speeds quoted “up to” a rate the port cannot reach. A Gen 2 drive in a Gen 1 port runs at Gen 1 speed — the port, not the drive, sets the ceiling.
- USB-C mentioned without the protocol. USB-C is a connector shape; the protocol inside can still be USB 2.0. Check the protocol label, not the plug.
- Cable capability ignored. Gen 2×2 needs a certified dual-lane cable; an older cable caps the link at Gen 2 speed regardless of the devices at both ends.
Which tier does each product line need
| Product | Sensible tier | Why |
|---|---|---|
| USB flash drive | USB 3.2 Gen 1 for general use; Gen 2 for premium lines | Flash read/write at Gen 1 already saturates most use cases; Gen 2 matters for large file transfers |
| Memory card | Interface class on the card (e.g. UHS-I / UHS-II, SD 2.0) plus the reader tier | The slower of card and reader wins; pair them deliberately |
| Portable SSD | USB 3.2 Gen 2 or Gen 2×2 | The whole point of an external SSD is bandwidth; 10 Gb/s is the credible minimum |
| USB hub / dock | Gen 1 upstream for data; Gen 2 where video or storage expansion is offered | Bandwidth is shared across downstream ports — check the split, not just the headline rate |
How to write it into a specification
Specify the tier by its signalling rate as well as its marketing name, and state the test conditions for the quoted speed: file sizes, controller behaviour over sustained writes, and the host system used. A supplier who can reproduce the number under stated conditions is quoting a measurement; one who cannot is quoting a label.
Then test the ceiling yourself: a large sequential copy to the device, on the port you will actually ship to, on a fresh unit. If the result is within the quoted band, the specification and the product agree — which is the only outcome that protects a programme six months later.
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