Debian RAID 1 Legacy Boot

raid.cfg

# The method should be set to "raid".
d-i partman-auto/method string raid
# Specify the disks to be partitioned. They will all get the same layout,
# so this will only work if the disks are the same size.
d-i partman-auto/disk string /dev/sda /dev/sdb

# Next you need to specify the physical partitions that will be used. 

d-i  partman-auto/expert_recipe string  \
    scc-raid ::                         \
        512 512 512 raid                \
            $primary{ }                 \
            method{ raid }              \
        .                               \
        51200 51200 51200 raid          \
            method{ raid }              \
        .                               \
        8192 8192 8192 raid       \
            method{ raid }              \
        .                               \
        51200 1000000000 -1 raid        \
            method{ raid }              \
        .                               \
# Last you need to specify how the previously defined partitions will be
# used in the RAID setup. Remember to use the correct partition numbers
# for logical partitions.
# Parameters are:
# <raidtype> <devcount> <sparecount> <fstype> <mountpoint> \
#          <devices> <sparedevices>
# RAID levels 0, 1, 5, 6 and 10 are supported; devices are separated using "#"
d-i partman-auto-raid/recipe string \
   1 2 0 ext2 /boot                 \
         /dev/sda1#/dev/sdb1        \
   .                                \
   1 2 0 ext4 /                     \
         /dev/sda2#/dev/sdb2        \
   .                                \
   1 2 0 swap -                     \
         /dev/sda3#/dev/sdb3        \
   .                                \
   1 2 0 ext4 /home                 \
         /dev/sda4#/dev/sdb4        \
   .

# This makes partman automatically partition without confirmation.
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