System is Centos 5.6.
Warning in logwatch:-

--------------------- Smartd Begin ------------------------ 

 
Currently unreadable (pending) sectors detected:
	/dev/sda - 96 Time(s)
	53 unreadable sectors detected
 
Offline uncorrectable sectors detected:
	/dev/sda - 96 Time(s)
	53 offline uncorrectable sectors detected
 
---------------------- Smartd End ------------------------- 

Boot device is /dev/sda, /dev/sdb did not have correct boot sector.

Make /dev/sdb bootable:-

[optimax]# 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

[root@optiplex ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md1               19G  4.1G   14G  23% /
/dev/md3              303G  236G   53G  82% /data
/dev/md2              1.9G  827M 1011M  45% /var
/dev/md0               99M   37M   58M  39% /boot

Fail partitons:-

# mdadm --manage /dev/md0 --fail /dev/sda1
# mdadm --manage /dev/md1 --fail /dev/sda2

Remove from md arrays:-

# mdadm –manage /dev/md0 –remove /dev/sdb1

# mdadm --manage /dev/md1 --remove /dev/sdb2

Shutdown system and replace faulty disk. If your disks are hot swap, this step may not be needed.

  # shutdown -h now

Swap over faulty disk for new disk.

Copy partiton table from existing good disk to new disk, double check you are copying the right partition table to the right disk. Then check again :-}

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

Add partitons back in to md array:-

# mdadm --manage /dev/md0 --add /dev/sda1
mdadm: added /dev/sda1
# mdadm --manage /dev/md1 --add /dev/sda2
mdadm: added /dev/sda2

Watch the resync progress with:-

# watch cat /proc/mdstat

Install grub loader back on to new disk.

grub> find /grub/stage1
(hd0,0)
(hd1,0)

grub> device (hd0) /dev/sdb (or /dev/hdb for IDE drives)
grub> root (hd0,0)
grub> setup (hd0)
grub>quit