Over the years, USB had been upgraded and upgraded and upgraded and it’s easy to loose sight of how fast in theory and in practice a particular version of the USB standard could be. So here’s a quick cheat sheet with the theoretical maximum speed of different USB versions:
USB 2: 480 Mbit/s, ca. 0.5 Gbit/s, 60 Mbyte/s, in practice around 40 Mbyte/s with a hard drive.
USB 3.0: 5 Gbit/s, 400 Mbyte/s to applications, I’ve seen 180 Mbyte/s to a hard drive so far. This was the first USB standard that had sockets that were blue inside to distinguish it from the much slower USB 2, that had sockets that were black or yellow inside.
USB 3.1, Gen 2: 10 Gbit/s (Note: USB3.1 Gen 1 is limited to 5 GBit/s)
USB 3.2: 5, 10 and 20 Gibt/s depending on the number of wires used for the data transfer. USB 3.2 Gen 1 = 5 Gbit/s, USB 3.2 Gen 2 = 10 Gbit/s, USB 3.2 Gen 2×2 = 20 Gbit/s
USB 4.0: 40 Gibt/s depending on the number of wires used. So not every USB 4 capable device might support that speed. Update: In 2023, high end notebooks with USB 4.0 claim 40 Gbit/s support. Also: Thunderbird 4 seems to be the same as USB 4.0, Intel style.