According to me, if your computer is not copying your files to Flash USB, then there might be some problem with your USB. I would suggest you to format it by following the various steps:
>> Insert your pendrive in your computer port.
>> Make sure you do not open or format it directly.
>> Click on the option “Manage” by right clicking on “My Computers”.
>> There you will a computer management window, click on the option “Disk management”.
>> Select the disk which you want to format, by right click on the removable disk.
>> Then click on the option “Format”.
Hope this might fix your issue.