live usb - How to format a USB drive for Startup disk?

I want to make a bootable live USB from Ubuntu 14.04. When I insert a free USB, Startup Disk Creator says Capacity: 7.5GB; Free 0.0GB.

This is a free and working USB, as I can copy file on it. I tried to format it several times with GParted and Disk, but still the same result.

When I hit Erase in Startup Disc Creator, it gives the following error:

org.freedesktop.DBus.Python.gi._glib.GError: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/dbus/service.py", line 707, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/usr/share/usb-creator/usb-creator-helper", line 239, in Format
    block.call_format_sync('dos', GLib.Variant('a{sv}', {'erase': GLib.Variant('s', '')}), None)
gi._glib.GError: GDBus.Error:org.freedesktop.UDisks2.Error.Failed: Error synchronizing after initial wipe: Timed out waiting for object

How can I safely format and create a live USB?