guys i can't copy & paste file from computer to usb it's says
i already tried gksudo nautilus, and open as administrator.
gksudo nautilus won't work, since elementary OS does not come with Nautilus file manager.
But there's an equivalent to that. Go to Applications, right click on Files, select New Window as Administrator, and enter your password. Then try pasting the file.
From what I can see, you are trying to paste onto an ISO image or a bootable USB drive formatted for system installation. Can you try using another USB drive and see if you can paste files on it?