Retrying the installer seems to be your best option at this point. Note that ubiquity has a few known issues, please read
https://wiki.ubuntu.com/DapperReleaseNotes/UbiquityKnownIssues -- install stop https://launchpad.net/bugs/53893 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
