Solved: Problem with grub-install (d-i, soft RAID6, Jessie)

["W. Martin Borgert" <debacle@debian.org>]

Many thanks to Christian, David, and Pascal!
With your hints I was able to solve the problems.

Lessons learned:

 1. d-i needs to be in "low" questions mode (expert) to allow
    manual selection of partition type (e.g. DOS/MBR or GPT).

 2. d-i seems to call grub-install on /dev/md0, even if user
    explicitely selects /dev/sdXN - one need to select the
    manual option and type the right device names. Or go to
    shell and grub-install --force manually.

 3. Changing the partition type of one disk in RAID6 is easy: If
    all disks are OK, it is relatively safe to just remove one
    disk (mdadm fail + remove), copy the partition table from
    another disk (in case of MBR it can be done with sfdisk) and
    add the removed disk again. It gets synced very quickly.

Cheers