Although title is the same problem is different. I've installed Ubuntu 14.04 Trusty on my HP EliteBook 6930p Notebook. And I was trying to create a bootable usb for my DELL XPS 15z. After successful creation of usb with usb-creator-gtk I plugged in bootable usb stick to DELL laptop. The error message was "Operation System not found". I tried several things but could not make it. When I check from working Ubuntu, I can see that usb is bootable and has files for installation.