Comment 2 for bug 1167366

Now, since the boot is happing in BIOS mode and the grub-installer is instructed to install bootloader into "mbr" and it picks the wrong target, we can instead instruct a bootdev to the grub-installer. One way to achieve this is to preseed:

d-i grub-installer/only_debian boolean false
d-i grub-installer/with_other_os boolean false
d-i grub-installer/bootdev string /dev/mapper/isw_eaijafbabd_Volume0

(do not preseed any other grub-installer/* nor lilo-installer/*)

This is not generic, however as another machine will have a different id of the raid array.
To circumvent that, one can rely on the implementation detail that the first raid array will be a symlink to /dev/dm-0.
Thus using:

d-i grub-installer/only_debian boolean false
d-i grub-installer/with_other_os boolean false
d-i grub-installer/bootdev string /dev/dm-0

(do not preseed any other grub-installer/* nor lilo-installer/*)

I managed to boot the installer and complete the installation unattended in BIOS (Legacy) mode.

This is a workaround. The bug as far as I currently understand is inside grub-installer package which incorrectly suggests wrong device when preseeding grub-installer options only_debian & with_other_os as true, while performing dmraid installation.