I successfully installed and used Octave from a USB thumb drive for a Toshiba 205 netbook. Everything was fine. Then I quit and re-booted. Octave had disappeared from the launcher. And, I can't find relevant files in, say, the archive director or any other directory. Repeated. Same result. Please advise.
Is it a Live USB disk or did you install Ubuntu to the USB disk? If it's a live disk, be sure to reserve extra space or the drive will "reset" on each boot.