Okay, so I'm using a Vaio notebook(PCG-21313M), Windows 7, with no CD drive. I've put Ubuntu 12.04.1 onto a usb stick using pendrivelinux.com, and tried to follow instructions on that site to boot Ubuntu from the usb, but when I cycle to 'External HDD' when rebooting it just flashes 'Operating System missing' and all I can do from this point is power off. I'm very new to this and have no clue how to go about correcting this.