Is this a 'requirement' of this program that you place a copy of the license to a flash drive? If so, tell us more about this program, what is it, and where this instruction is.
To be truthful I don't see why it would be necessary to do this, and in particular to saving it to the Root Directory.
What Operating System are you using?
In Windows, the Windows installation is usually installed on the C:\ drive in a folder called "Windows". This C:\ drive is called the Root Directory. If you are using Windows you can check your own Root Directory by using Windows Explorer, (My Computer will do), and highlighting the C drive. That will list all folders and files stored on the Root Directory.
So, if you connect up a Flash Drive to the computer and explore its contents, you will see the drive given a drive letter by Windows, (E:\ seems right, but all systems are different), and will see what folders and files are already stored on it. The Root Directory for the flash drive will be E:\ so all you need to do is copy, or drag and drop, that file directly onto the E drive.
Hope that helps.