What box are you trying to flash?
Have you got the correct build, many people get the wrong one.
Is your drives formatted to fat 32, most say you have to low level format but I haven't had to ever.
Make sure there is only the zgemma folder on your drive, if it's a h2s or hs then in the zgemma folder should be another folder called h3, if it's the 2or 2s then in the zgemma folder should be sh1.
If it's h2s And hs it should flash from the front port, from other boxes try the rear port .
The front LCD should display Flsh, if it says Usb press power on box.
R.T.F.I