New ext drive not seen for destination drive

[Ronea2]

I'm attempting to to use a Windows 8.1 machine to create a Windows 7 boot drive on a new Western Digital  1tb external hard drive.  While using Wintousb, all goes well until i get to the screen where I should select my destination drive.  At that point, my external hard drive does not show up - even though I can see it in explorer.  I'm not sure what I'm doing wrong.

Thanks,

Ron