For anyone else hitting this error, a workaround (if you truly don't need anything on that partition) is to click on the "format" checkbox in the "Prepare partitions" page.
-- Crash if old install exists on harddrive https://launchpad.net/bugs/89461 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
