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.