I don't know why, but GParted could still see it and the end result was still a bootable USB drive, so I guess it doesn't really matter.) (While testing this I couldn't mount the USB drive anymore after setting the boot flag. In GParted, right click the partition, choose "manage flags" and then check the "boot" option.
In GParted, chose "Device" and then "Create partition table.". Create a GPT partition table on your USB drive.This is what I do to create a bootable USB drive for UEFI firmware: Don't know why, but WinUSB worked so I didn't investigate further.)
(Edit: I just tried this with Windows 10 without success. I've successfully done this with both Windows 8.1 and Ubuntu 14.04, but I can't vouch for any other OS. The ISO must be configured for UEFI boot for this to work. CSM)) all you'll need is GParted and a file manager.
If you boot with UEFI (not BIOS or UEFI with BIOS compatibility mode (a.k.a.