The read and write speed, which you get from the tools, depends on the USB port to which the flash drive is connected (USB 2.0 or USB 3.0), the configuration of your PC, and the size of the file you are transferring to the flash drive. If you are testing a USB 3.0 flash drive, then you should connect it to a USB 3.0 port to get the right transfer speed.
Before using any tool, you can check USB Settings, let me show you an effortless way to get a glimpse of the average write speed of your flash drive. On Windows 8 and above, the copying window will show you the average write speed in real time.
Here are some of the tools that would help you in testing the data transfer speed of your USB flash drive:
1. H2testw (Free | Windows)
A free tool to check for errors and report read and write speed within the results. Check Flash writes a temporary file to the flash drive and test for the errors and read-write speed. You can set the tool to check the whole drive, the number of cycles until the error is found. The tool is free to download and does not need installation to work.
2. CrystalDiskMark (Free | Windows)
CrystalDiskMark is a disk benchmark software which comes with both installers as well as portable mode. This software is a multitasker which can also test the RAM for reading and write speed. The user interface is clear and simple with big numbers showing the current write speed. CrystalDiskMark comes with different themes if you care for them.
3. Disk Speed Test (Free | macOS 10.6+)
Disk Speed is a free app for Mac OS 10.6 and higher to test the read and write speed for USB drives. The user interface is straightforward with reading and writes speed meters on the top and detailed results below. Disk Speed Test mainly writes a series of videos to the drive to check the speed from PAL, NTSC to 4k videos and record the speed. You can also save a screenshot of the result for comparing two flash drives.