Comment 9 for bug 743136

I think this would actually be best fixed in your custom-partitioner udeb, although it was triggered by a change in mdadm in natty. Compare the corresponding fixes that we made to our standard partitioner:

  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=591917
  http://bazaar.launchpad.net/~ubuntu-installer/partman-md/master/revision/1016

Could you try the attached patch, which should work in older releases as well as in natty?