Booting with a failed primary HDD on a Linux RAID1 array

Thought it would be straightforward, and that the system would automagically take care of it by itself, think again:

1. GRUB is dumb, until we tell it exactly what to do, so, assuming your /boot partition is located on sda1/sdb1, you need to make sure that you manually setup both disks, using:
   

   [root@whatever ~]# grub
   
       GNU GRUB  version 0.95  (640K lower / 3072K upper memory)
   
    [ Minimal BASH-like line editing is supported.  For the first word, TAB
      lists possible command completions.  Anywhere else TAB lists the possible
      completions of a device/filename.]
   
   grub> root (hd0,0)
    Filesystem type is ext2fs, partition type 0xfd
   
   grub> setup (hd0)
    Checking if "/boot/grub/stage1" exists... yes
    Checking if "/boot/grub/stage2" exists... yes
    Checking if "/boot/grub/e2fs_stage1_5" exists... yes
    Running "embed /boot/grub/e2fs_stage1_5 (hd0)"...  15 sectors are embedded.
   succeeded
    Running "install /boot/grub/stage1 (hd0) (hd0)1+15 p (hd0,0)/boot/grub/stage2
   /boot/grub/grub.conf"... succeeded
   Done.
   
   grub> root (hd1,0)
    Filesystem type is ext2fs, partition type 0xfd
   
   grub> setup (hd1)
    Checking if "/boot/grub/stage1" exists... yes
    Checking if "/boot/grub/stage2" exists... yes
    Checking if "/boot/grub/e2fs_stage1_5" exists... yes
    Running "embed /boot/grub/e2fs_stage1_5 (hd1)"...  15 sectors are embedded.
   succeeded
    Running "install /boot/grub/stage1 (hd1) (hd1)1+15 p (hd1,0)/boot/grub/stage2
   /boot/grub/grub.conf"... succeeded
   Done.
   
   grub> quit

   
   
2. If you have additional disks that you fsck (last columns of your fstab), be mindful that libsata will reorganize their names around, so if you set your /dev/sdc1 to be checked at bootime, you might end up getting stuck with fsck not finding the disk, and not being able to boot at all!