Comment 4 for bug 781587

Are you installing from a memory stick or physical CD-ROM? I have found in the past on the new Macbook Air that something resets USB memory sticks during Ubuntu installation and they apparently vanish to the installer or at least stop working. Not entirely surprising because I've seen kernel developers working with and trying to fix some unexpected USB-reset issues on the new Macbook Airs.

Make sure you're using a USB CD-ROM drive. That seems to avoid the problems I have seen. And if you are already installing from a CD-ROM then please try burning another copy (at low speed) to ensure its correctness.