First, confirm the USB is recognized outside the tool. IF you were to go to My Computer, do you see it?
Yes? Then next step,
I usually recommend Windows 7 USB Download Tool, bc it breaks down each step clearly, and it looks clean. When it fails, I use Rufus.
Download link: http://rufus.akeo.ie/
Just choose device it should recognize your Sandisk, then the Windows 7 iso. Click Start.
TIP: Make sure you have .NET Framework installed. Cheers