What's the maximum typical speed possible with a USB2.0 drive?

USB 2 uses 1 millisecond frames, and in High Speed (480 Mb/s) mode they are divided into 8 micro-frames. The maximum size of bulk packets (used by USB mass storage devices) is 512 bytes. According to this very informative document the theoretical maximum is 13 packets per microframe. So the theoretical maximum speed of a USB 2 drive is:

1000 * 8 * 512 * 13 = 53248000 ~= 53 MB/s

This other document from Cypress says near the end that they actually acheive 43 MB/s.

In practice the limit will usually be the flash itself.

Edit: This information is actually also in the USB 2 spec.

Your flash drive is the bottleneck. They can't reach the 60 MB/s theoretical maximum. Here's an excerpt from Wikipedia:

Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s (60MB/s) which the USB 2.0 Hi-Speed specification supports because of technical limitations inherent in NAND flash. The fastest drives currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput. (...)

Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that speed. This is about 20 times faster than USB 1.1 "full speed" devices which are limited to a maximum speed of 12 Mbit/s (1.5 MB/s).

Around 30 MB/sec is quite typical maximum transfer speed.

USB 1.0 and USB 2.0 connections are half-duplex, meaning data flows in only one direction at a time. Shared connection between both directions is probably biggest reason for slowdown than expected transfer speed.

In comparison, USB 3 and Ethernet are full duplex and do meet expected transfer speeds better.

In my machine, an USB2 flash drive speed never exceeds 33 MB/s in test application, even though Windows reported 33-37 MB/s speed. I did some testing and enabled disk cache (device properties) and increased usb max transfer size to 2 MB (KB2581464) but could not make it any faster.

The USB 2.0 interface can be a limit due to signalling and command overhead as well as spacing between packets.

I have a fast SSD connected by USB 2.0. The drive is much faster than the interface (by more a factor of 10).

Read Speed maxes out around 33 MB/s and Write Speed at 17.5 MB/s. Write Speeds are almost 50% slower due to a verify-read after the write and the fact that the USB signal is half-duplex as another answer mentions.

With an iMac mid-2007 and one Verbatim USB2 disk transferring data to a FW800 drive I get 36-37 MB/s. It's already very good for USB2.

If I add a second transfer from another USB2 disk (Packard Bell) connected to the same USB2 hub to the same FW800 drive, the combined transfer rate increases to 42 MB/s. This is exceptional and it's the highest transfer rate I have ever seen on USB2.

More than 35-40 MB/s on USB2.0 is practically impossible and I was already dedicating a USB2 controller only for those disks, no mouse or other devices interfering.