It took me some time to realize that this was the solution for my RAID preseed problem as well:
Bug #591909 -- partman-auto creates faulty partition table on netboot with preseed https://bugs.launchpad.net/bugs/574232 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
