Check your USB for errors with this tool. H2testw is a simple and easy to use program designed to enable you to test USB sticks for various kinds of errors.
It can also be used for any other storage media like memory cards, internal and external hard drives and even network volumes.

H2testw was developed to test USB sticks for various kinds of errors. It can also be used for any other storage media like memory cards, internal and external hard drives and even network volumes. To run the test for your drive, just plug in the drive, format it in Windows, run H2testw, select the target, and click Write + Verify button. H2testw writes files of up to 1GB to the chosen destination and names them 1.h2w, 2.h2w, 3.h2w and so on. If the target directory already contains such a set of files H2testw will offer to verify them. After it is done the software leaves its test files on the medium. You can erase them if you like or verify them again.

If the drive that you tested passes the test of H2testw without errors, then the advertised capacity is correct (minus a small overhead for operating system file formatting). However, if you are seeing a sample output like below, then it is a counterfeit. This is 4GB USB flash drive not a 64GB USB flash drive. As 4GB drive it should be about 3.9 GB not 3.8 GB formatted. This indicates in low level formatting a lot of bad sectors had to be locked out. Usually flash chips like these are rejected for major brands but somehow is being resold and reprogrammed.

The license of this software is Freeware, you can free download and free use this diagnostic software.