I am aware of the partman-md/confirm_nooverwrite thing, that was the solution to most of this problems in the past. The thing is, that the preseed-file works for almost all hardware-platforms I tested, but only for one (Primergy MX130) it keeps asking for confirmation, before writing the partition-layout to the disks. All machines I tested are running with two SATA Disks, nothing special.

I'm not really sure, what information could be needed in order to investigate the cause of this behaviour, but I would of course be willing to provide more information, if someone has an idea.

Relevant part of the preseed file is the following:

    d-i partman-auto/disk string /dev/sda /dev/sdb
    d-i partman-auto/method string raid

    d-i partman-md/confirm boolean true
    d-i partman-partitioning/confirm_write_new_label boolean true
    d-i partman-md/device_remove_md boolean true
    d-i partman/choose_partition select finish
    d-i partman-md/confirm_nooverwrite boolean true

    # Write the changes to disks?
    d-i partman/confirm boolean true
    d-i mdadm/boot_degraded boolean true

    # RECIPE
    # Next you need to specify the physical partitions that will be used. 
    d-i partman-auto/expert_recipe string \
          multiraid :: \
                  500 10000 1000000000 raid $lvmignore{ }\
                          $primary{ } \
                          method{ raid } \
                  . \
                  512 1000 786 raid $lvmignore{ }\
                          $primary{ } \
                          method{ raid } \
                  .  \
                  8192 10240 10240 raid $lvmignore{ }\
                          method{ raid } \
                  .

    # Parameters are:
    # <raidtype> <devcount> <sparecount> <fstype> <mountpoint>  <devices>         <sparedevices>
    d-i partman-auto-raid/recipe string \
        1 2 0 ext4 / /dev/sda1#/dev/sdb1 . \
        1 2 0 ext2 /boot /dev/sda2#/dev/sdb2 . \
        1 2 0 swap - /dev/sda5#/dev/sdb5 .