Preseed ignores existing OS

[Thomy23]

I have a problem with preseeding Ubuntu 9.04 (based on a an alternate disk and a custom preseed file). I want to use the free part of the disk for my ubuntu installation as there's already windows installed and i want to use dualboot. My preseed looks like this:

d-i partman-auto/init_automatically_partition select biggest_free
d-i partman-auto/method string regular
d-i partman-auto/choose_recipe select home
d-i partman/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i grub-installer/with_other_os boolean true

But after the installation, the windows partition is gone and grub has been written to the MBR. It seems to me like partman doesn't pay attention to the existing partition (and afterwards grub can't recognize windows because the partition has been wiped). I used the standard Ubuntu 9.04 alternate cd for preseeding.

Greetings

Thomas