I have a RAID1 array on my Ubuntu 12.04 LTS and my /sda HDD has been replaced several days ago. I use this commands to replace:

# go to superuser
sudo bash
# see RAID state
mdadm -Q -D /dev/md0
# State should be "clean, degraded"
# remove broken disk from RAID
mdadm /dev/md0 --fail /dev/sda1
mdadm /dev/md0 --remove /dev/sda1
# see partitions
fdisk -l
# shutdown computer
shutdown now
# physically replace old disk by new
# start system again
# see partitions
fdisk -l
# copy partitions from sdb to sda
sfdisk -d /dev/sdb | sfdisk /dev/sda
# recreate id for sda
sfdisk --change-id /dev/sda 1 fd
# add sda1 to RAID
mdadm /dev/md0 --add /dev/sda1
# see RAID state
mdadm -Q -D /dev/md0
# State should be "clean, degraded, recovering"
# to see status you can use
cat /proc/mdstat

This is the my mdadm output after sync:

/dev/md0:
        Version : 0.90
  Creation Time : Wed Feb 17 16:18:25 2010
     Raid Level : raid1
     Array Size : 470455360 (448.66 GiB 481.75 GB)
  Used Dev Size : 470455360 (448.66 GiB 481.75 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Nov  1 15:19:31 2012
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 92e6ff4e:ed3ab4bf:fee5eb6c:d9b9cb11
         Events : 0.11049560

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1

After bebuilding completion "fdisk -l" says what I have not valid partition table /dev/md0. This is my fdisk -l output:

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 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: 0x00057d19

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *          63   940910984   470455461   fd  Linux raid autodetect
/dev/sda2       940910985   976768064    17928540    5  Extended
/dev/sda5       940911048   976768064    17928508+  82  Linux swap / Solaris

Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 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: 0x000667ca

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          63   940910984   470455461   fd  Linux raid autodetect
/dev/sdb2       940910985   976768064    17928540    5  Extended
/dev/sdb5       940911048   976768064    17928508+  82  Linux swap / Solaris

Disk /dev/md0: 481.7 GB, 481746288640 bytes
2 heads, 4 sectors/track, 117613840 cylinders, total 940910720 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: 0x00000000

Disk /dev/md0 doesn't contain a valid partition table

This is my grub install output:

root@answe:~# grub-install /dev/sda
/usr/sbin/grub-setup: warn: Attempting to install GRUB to a disk with multiple partition labels or both partition label and filesystem.  This is not supported yet..
/usr/sbin/grub-setup: error: embedding is not possible, but this is required for cross-disk install.
root@answe:~# grub-install /dev/sdb
Installation finished. No error reported.

So

1) "update-grub" find only /sda and /sdb Linux, not /md0

2) "dpkg-reconfigure grub-pc" says "GRUB failed to install the following devices /dev/md0"

I cannot load my system except from /sdb1 and /sda1, but in DEGRADED mode...

Anybody can resolve this issue? I have big headache with this.