mdadm --detail /dev/md0  # to identify the parts

  umount /boot
  mdadm --stop /dev/md0
  mkdir -p /mnt/a /mnt/b
  mount -o ro -t ext2 /dev/hde1 /mnt/a
  mount -o ro -t ext2 /dev/hdg1 /mnt/b
  cat /mnt/a/grub/default  # identify desired one, in my case this one
  cat /mnt/b/grub/default  # identify desired one
  umount /mnt/a
  umount /mnt/b
  rmdir /mnt/a /mnt/b

  # No reason not to have it online.
  mdadm --assemble /dev/md0
  mount /boot

  # Force a resync.
  mdadm /dev/md0 --fail /dev/hdg1
  mdadm /dev/md0 --remove /dev/hdg1
  mdadm /dev/md0 --add /dev/hdg1
  cat /proc/mdstat

Also

  mdadm /dev/md0 --fail /dev/hdg1
  mdadm /dev/md0 --remove /dev/hdg1
  mdadm /dev/md0 --add /dev/hdg1

todo http://lists.us.dell.com/pipermail/linux-poweredge/2003-July/008898.html
  howto install grub on second raid mirror

todo

I just learned something a little disconcerting.  Every so often you
need to manually update grub stage files.

  tesuji:~# cp -a /boot/grub /boot/grub2

  tesuji:~# grub-install "(hd0)"

  tesuji:~# diff -r --brief /boot/grub2 /boot/grub
  Files /boot/grub2/default and /boot/grub/default differ
  Files /boot/grub2/e2fs_stage1_5 and /boot/grub/e2fs_stage1_5 differ
  Files /boot/grub2/fat_stage1_5 and /boot/grub/fat_stage1_5 differ
  Files /boot/grub2/jfs_stage1_5 and /boot/grub/jfs_stage1_5 differ
  Files /boot/grub2/minix_stage1_5 and /boot/grub/minix_stage1_5 differ
  Files /boot/grub2/reiserfs_stage1_5 and /boot/grub/reiserfs_stage1_5 differ
  Files /boot/grub2/stage2 and /boot/grub/stage2 differ
  Files /boot/grub2/xfs_stage1_5 and /boot/grub/xfs_stage1_5 differ

I found this because I had a mirrored /boot that had differing files
there (written to under the mirror and the mirror was out of sync) and
was trying to figure out which was correct when I bumped into this
issue.  Looking at it further I find the following info listed in
/usr/share/doc/NEWS.Debian.gz:

  In order to prevent that, it is recommended that you install the
  latest GRUB by issuing (as root) the following command after
  upgrading the grub package:

    grub-install "(hd0)"

So...  It appears that just taking grub updates with apt-get upgrade
isn't enough to actually keep all of the files up to date.  You need
to manually run grub-install to do that part of the upgrade.  I had
machines installed from waaaayy baaacckkk and found some very old
copies of the stage files.  The dates on the files represent the time
that grub-install has last updated those files.

Now this probably isn't really a serious problem.  But it is one of
the areas that an upgraded system differs from a freshly installed
system.  I think those should be minimized when machines are
upgraded.  And there is a potential for strange problems due to older
files no longer being tested with newer kernels.