Comment 1 for bug 803749

Also seeing this issue as well but only when i try and preseed the disk partitions, if i manually enter in the partitions things work OK

d-i partman-auto/disk string /dev/sda /dev/sdb /dev/sdc /dev/sdd
d-i partman-auto/expert_recipe string multiraid :: 989999 9899999 989999 raid $primary{ } $bootable{ } method{ raid } . 1 1 1 raid method{ biosgrub } . 10000 10000 10000 raid method{ raid } .
d-i partman-auto-raid/recipe string 1 2 0 ext4 / /dev/sda1#/dev/sdb1 . 1 2 0 swap - /dev/sda5#/dev/sdb5 . 1 2 0 ext4 /app1 /dev/sdc1#/dev/sdd1 .
d-i partman-basicfilesystems/mountoptions multiselect noatime
d-i mdadm/boot_degraded boolean true
d-i partman-md/confirm boolean true
d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman-md/confirm_nooverwrite true

Following Bug #743136 looks simular so could be a regression some where.

Work around is to drop into the shell and run the following

chroot /target
ln -s /dev/md0 /dev/md/0
ln -s /dev/md1 /dev/md/1
ln -s /dev/md2 /dev/md/2
grub-install /dev/sda
grub-install /dev/sdb