Debian Sarge (and Woody)
Ubuntu

Problem:
--------
Root is installed on raid but after reboot other drive is always missing and
logs looks like this:

***************************************************
md: md0 stopped.
md: bind<hdb3>
raid1: raid set md0 active with 1 out of 2 mirrors
***************************************************

If you use Debian Sarges installer to create root on raid you will encounter
this problem. Also migrating from non-raid root may lead you to this
problem. If you build degraded array (mdadam /dev/md0 --create -l1 -n2
/dev/hda? missing) and install kernel-image to raid-root while it's degraded
you will end up missing other disk after every reboot even if you always
hotadd it to array.

Problem lies in mkinitrd, following information is from it's manpages: "If
both mdadm(8) and raidtools2 are installed, the former is preferred.  At the
moment, mkinitrd uses the -D option of mdadm(8) to discover the constituent
devices. This means that only devices that are part of the array at the time
that mkinitrd is run will be used later on. This problem does not exist when
raidtools2 is used."


Solution:
---------
Reinstall kernel-image (or re-create initrd.img) after you have hot-added
partition that raid losed in boot (and wait it to resync). This way mkinitrd
creates a new initrd.img which contains both partitions for raid1.