Using physical devices is not what you want to do when creating new guests. Use an ISO image that you can get from Ubuntu Download site. They are available for Linux as Torrents and thus, even if your internet connection is slow and breaks up every once in a while, you will still get 100% correct image in the end - obviously just slower. Just use google to find the place.
You can also create the .iso image from the dvd that you have. You don't say what your host OS is - in Linux it is simply done dumping it out with dd-command. In Windows, it's an utilty. If there are defects in the disk, as there easily is, then reading the disk isn't always working, not easily anyway.
Anyway, download is the easiest option.