The GRUB boot loader was previously installed to a disk that is no longer present, or whose unique identifier has changed for some reason. It is important to make sure that the installed GRUB core image stays in sync with GRUB   │ │ modules and grub.cfg. Please check again to make sure that GRUB is written to the appropriate boot devices.                                                                                    If you're unsure which drive is designated as boot drive by your BIOS, it is often a good idea to install GRUB to all of themhem.

Note: it is possible to install GRUB to partition boot records as well, and some appropriate partitions are offered here. However, this forces GRUB to use the blocklist mechanism, which makes it less reliable, and therefore is   │ │ not recommended.
GRUB install devices:

[ ] /dev/sda (1500301 MB; ???)     [ ] /dev/sdb (1500301 MB; ???)       [ ] /dev/md2 (1497617 MB; ???)  [ ] /dev/md1 (536 MB; ???)

$ sudo fdisk  -l /dev/sda

Disk /dev/sda: 1500.3 GB, 1500301910016 bytes
64 heads, 32 sectors/track, 1430799 cylinders, total 2930277168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000bab3b

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     4194303     2096128   fd  Linux raid autodetect
/dev/sda2         4194304     5242879      524288   fd  Linux raid autodetect
/dev/sda3         5242880  2930276351  1462516736   fd  Linux raid autodetect



$ sudo fdisk  -l /dev/sdb

Disk /dev/sdb: 1500.3 GB, 1500301910016 bytes
64 heads, 32 sectors/track, 1430799 cylinders, total 2930277168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000f16e2

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048     4194303     2096128   fd  Linux raid autodetect
/dev/sdb2         4194304     5242879      524288   fd  Linux raid autodetect
/dev/sdb3         5242880  2930276351  1462516736   fd  Linux raid autodetect



$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md2              1.4T   86G  1.2T   7% /
none                  5.9G  236K  5.9G   1% /dev
none                  5.9G     0  5.9G   0% /dev/shm
none                  5.9G   60K  5.9G   1% /var/run
none                  5.9G     0  5.9G   0% /var/lock
/dev/md1              496M   98M  373M  21% /boot



$ cat /etc/mdadm/mdadm.conf
DEVICES /dev/[hs]d*
ARRAY /dev/md0 UUID=ca2db9aa:610f4882:776c2c25:004bd7b2
ARRAY /dev/md1 UUID=ed4f5082:6ff10c9e:776c2c25:004bd7b2
ARRAY /dev/md2 UUID=4f1df183:61f9d166:776c2c25:004bd7b2
MAILADDR root


$ sudo  mdadm --detail /dev/md1 --detail
/dev/md1:
        Version : 0.90
  Creation Time : Thu Sep  1 19:36:00 2011
     Raid Level : raid1
     Array Size : 524224 (512.02 MiB 536.81 MB)
  Used Dev Size : 524224 (512.02 MiB 536.81 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Tue Jan  3 18:05:29 2012
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : ed4f5082:6ff10c9e:776c2c25:004bd7b2
         Events : 0.20

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2
nomad@taw-eu1:~$ sudo  mdadm --detail /dev/md2 --detail
/dev/md2:
        Version : 0.90
  Creation Time : Thu Sep  1 19:36:00 2011
     Raid Level : raid1
     Array Size : 1462516672 (1394.76 GiB 1497.62 GB)
  Used Dev Size : 1462516672 (1394.76 GiB 1497.62 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 2
    Persistence : Superblock is persistent

    Update Time : Tue Jan  3 18:23:57 2012
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 4f1df183:61f9d166:776c2c25:004bd7b2
         Events : 0.23

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       1       8       19        1      active sync   /dev/sdb3
nomad@taw-eu1:~$ cat /proc/mdstat
Personalities : [raid1] [raid0] [raid6] [raid5] [raid4] [raid10] [linear] [multipath]
md2 : active raid1 sdb3[1] sda3[0]
      1462516672 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      524224 blocks [2/2] [UU]

md0 : active raid1 sdb1[1] sda1[0]
      2096064 blocks [2/2] [UU]

unused devices: <none>