You boot up with a live cd not from any distro installed already onto a drive. A good article referring to Knoppix seen in 2004 can be applied to ubuntu with the title "Computer First Aid using Knoppix" seen at
Computer First Aid Using Knoppix
The screenshot there was while booted up with the latest release for ubuntu live to show how easy it was to grab files even from an external usb drive and save them to folders on a Vista or XP primary.
The live cd installation option to a drive won't detect the other OSs installed on other drives while the install only download to install might. For some reason other drives are not seen. The live cd generally works best since that's a self contained OS for cross platform access like some releases of GParted seem to be.