Comment 4 for bug 1423605

You should be able to work around this with something like:
--- /etc/maas/preseeds/curtin_userdata.dist 2015-03-25 19:39:19.667272991 +0000
+++ /etc/maas/preseeds/curtin_userdata 2015-03-25 19:39:21.591291807 +0000
@@ -12,6 +12,7 @@
   driver_02_add: ["add-apt-repository", "-y", "deb {{driver['repository']}} {{node.get_distro_series()}} main"]
   driver_03_update_install: ["sh", "-c", "apt-get update --quiet && apt-get --assume-yes install {{driver['package']}}"]
   driver_04_load: ["sh", "-c", "depmod && modprobe {{driver['module']}}"]
+ wipe_mdadm: ["sh", "-c", 'apt-get install -qy mdadm; for i in /dev/md*; do [ -b "$i" ] || continue; mdadm --stop "$i"; done; udevadm settle; for i in /dev/[xsv]d*; do [ -b "$i" ] || mdadm --zero-superblock "$i"; done']
 {{endif}}
 late_commands:
   maas: [wget, '--no-proxy', '{{node_disable_pxe_url|escape.shell}}', '--post-data', '{{node_disable_pxe_data|escape.shell}}', '-O', '/dev/null']