This is probably bug 58688. Please attach your /var/log/installer/syslog, /var/log/syslog, and /var/log/partman so that I can be sure.
** Changed in: ubiquity (Ubuntu) Status: Unconfirmed => Needs Info -- Install failure Dapper, on external firewire disk https://launchpad.net/bugs/58720 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs