External USB hard drives - what speeds should be expected?

Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet.[62] This is 70% of the total bandwidth available. 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. Overhead is a component of all connectivity standards." For isochronous devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission.

USB supports the following signaling rates: The terms speed and bandwidth are used interchangeably. "high-" is alternatively written as "hi-".

A low-speed rate of 1.5 Mbit/s (~183kB/s) is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks. The full-speed rate of 12 Mbit/s (~1.43 MB/s) is the basic USB data rate defined by USB 1.1. All USB hubs support full-bandwidth.

A high-speed (USB 2.0) rate of 480 Mbit/s (~57 MB/s) was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; i.e. they are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.

A SuperSpeed (USB 3.0) rate of 4800 Mbit/s (~572 MB/s). The written USB 3.0 specification was released by Intel and partners in August 2008. The first USB 3 controller chips were sampled by NEC May 2009[55] and products using the 3.0 specification arrived beginning in January 2010.[56] USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.

Source: http://en.wikipedia.org/wiki/Universal_Serial_Bus

Ubuntu's Disk Utility program has the ability to benchmark a drive, with nice graphs and data. You can test the read speed on a drive in use, but I think the drive needs to be unpartitioned and unformatted for it to run a read + write benchmark test.

I normally see about 20-25MB/s when using external USB drives, it usually won't go much higher, but it can drop quite a bit, and sometimes hang.

I have several external hard drives and speaking from experience I have attained speeds of up to 20Mbp/s, all of which are connected via USB 2.0.

This has been the same speeds attained when I was using Windows also, and as there's been no increase or decrease since I changed to Ubuntu, I'd guess that the drives were running at their optimal.

As for the question of testing the transfer rate of a USB hard drive, I'm unsure what benchmarking software is available on Ubuntu.