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.