What is the typical USB 2.0 external hard drive speed ?
Typical hi-speed USB 2.0 hard drives can be written to at rates around 25-30 MB/s, and read at 30-40 MB/s. This represents ~70% of the total available bandwidth.
According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead - the communication protocol between the card and the peripheral.
Typical SuperSpeed USB 3.0 external hard drives can transfer large files at ~70 MB/s
The USB standard supports the following speeds:
USB 1.0 - 1.5 Mbit/s (~183kB/s) - intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks.
USB 1.1 - 12 Mbit/s (~1.43 MB/s) - all USB 1.1 hubs support full-bandwidth.
USB 2.0 (High-Speed, a.k.a. Hi-Speed) - 480 Mbit/s (~57 MB/s) - all devices are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.
USB 3.0 (SuperSpeed) - 4800 Mbit/s, 4.8Gbits/s (~572 MB/s) - connectors are generally backwards compatible, but include new wiring and full duplex operation.