Пришло сообщение, что отказал RAID:

  This is an automatically generated mail message from mdadm running on
  A DegradedArray event had been detected on md device /dev/md0. 
  Faithfully yours, etc.
  P.S. The /proc/mdstat file currently contains the following:
  Personalities : [raid1]
  md0 : active raid1 sda2[1]
     388604224 blocks [2/1] [_U]
  unused devices: <none>



Смотрим, что случилось:

   #cat /proc/mdstat

   Personalities : [raid1]
   md0 : active raid1 sdb2[1]
     388604224 blocks [2/1] [_U]
   
   #mdadm --detail /dev/md0

   /dev/md0:
       Version : 00.90
    Creation Time : Thu Feb 19 14:59:47 2009
    Raid Level : raid1
    Array Size : 388604224 (370.60 GiB 397.93 GB)
    Used Dev Size : 388604224 (370.60 GiB 397.93 GB)
    Raid Devices : 2
   Total Devices : 1
   Preferred Minor : 0
   Persistence : Superblock is persistent

   Update Time : Thu Mar 12 11:23:38 2009
         State : active, degraded
   Active Devices : 1
   Working Devices : 1
   Failed Devices : 0
   Spare Devices : 0
   UUID : 60b0013f:7372c5d4:262fce52:c3c9a424
   Events : 0.321

   Number   Major   Minor   RaidDevice State
      0       0        0        0      removed
      1       8       18        1      active sync   /dev/sdb2

Вынимаем битый диск и подсовываем такой же по размеру, проверяем:

   # fdisk -l
   Disk /dev/sda: 400.0 GB, 400088457216 bytes
   Disk /dev/sda doesn't contain a valid partition table

   Disk /dev/sdb: 400.0 GB, 400088457216 bytes
   Device Boot      Start         End      Blocks   Id  System
   /dev/sdb1               1         262     2104483+  82  Linux swap / Solaris
   /dev/sdb2   *         263       48641   388604317+  fd  Linux raid autodetect

Создаем разделы, как на живом диске:

   # sfdisk -d /dev/sdb | sfdisk /dev/sda

Проверяем разбивку:

   # fdisk -l
   Disk /dev/sda: 400.0 GB, 400088457216 bytes
   Device Boot      Start         End      Blocks   Id  System
   /dev/sda1               1         262     2104483+  82  Linux swap / Solaris
   /dev/sda2   *         263       48641   388604317+  fd  Linux raid autodetect

   Disk /dev/sdb: 400.0 GB, 400088457216 bytes
   Device Boot      Start         End      Blocks   Id  System
   /dev/sdb1               1         262     2104483+  82  Linux swap / Solaris
   /dev/sdb2   *         263       48641   388604317+  fd  Linux raid autodetect

Теперь можно подключить новый диск в raid:

   # mdadm --add /dev/md0 /dev/sda2

   mdadm: added /dev/sda2

Проверяем как идет процесс ребилдинга:

   # cat /proc/mdstat
   Personalities : [raid1]
   md0 : active raid1 sda2[2] sdb2[1]
     388604224 blocks [2/1] [_U]
     [>....................]  recovery =  0.5% (1944960/388604224) finish=102.7min speed=62740K/sec

   unused devices: <none>

и не забыть про grub:

   # grub
   grub> root (hd1,0)
   grub> setup (hd1)
   grub> quit

И все готово.