Mitel SME server 7.5 running Centos 4.8
2x RAID1 (= hard disk mirror) volumes configured.
The first one is called /dev/md1 and is 106MB in size.
The second one is called /dev/md2 and is 79GB in size.

fdisk -l
Disk /dev/sda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      104391   fd  Linux raid autodetect
/dev/sda2              14        9729    78043770   fd  Linux raid autodetect

cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sda1[1]
104320 blocks [2/1] [_U]

md2 : active raid1 sda2[1]
78043648 blocks [2/1] [_U]

mdadm -D /dev/md1
/dev/md1:
Version : 00.90.03
Creation Time : Tue Feb  9 18:23:06 2010
Raid Level : raid1
Array Size : 104320 (101.89 MiB 106.82 MB)
Used Dev Size : 104320 (101.89 MiB 106.82 MB)
Raid Devices : 2
Total Devices : 1
Preferred Minor : 1
Persistence : Superblock is persistent

Update Time : Sat Oct 29 14:29:04 2011
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

UUID : 75033f39:83173411:7e4c09d0:950cf5d3
Events : 0.50

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

mdadm -D /dev/md2
/dev/md2:
Version : 00.90.03
Creation Time : Tue Feb  9 18:23:06 2010
Raid Level : raid1
Array Size : 78043648 (74.43 GiB 79.92 GB)
Used Dev Size : 78043648 (74.43 GiB 79.92 GB)
Raid Devices : 2
Total Devices : 1
Preferred Minor : 2
Persistence : Superblock is persistent

Update Time : Sat Oct 29 14:36:08 2011
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

UUID : e2d7cddf:055ea73c:0bdde8e9:f9e08fd4
Events : 0.67312

Number   Major   Minor   RaidDevice State
0       0        0        0      removed
1       8        2        1      active sync   /dev/sda2

Shut down the computer, temporarily disconnect the power cord, swap the faulty hard disk for a new hard disk and and power up the system.
We will now attempt to rebuild the RAID1 mirror hard disk set [‘syncing’].  This will make changes to the operating system.

fdisk -l
Disk /dev/sda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      104391   fd  Linux raid autodetect
/dev/sda2              14        9729    78043770   fd  Linux raid autodetect

Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/sdb doesn’t contain a valid partition table

fdisk /dev/sdb
The number of cylinders for this disk is set to 60801.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
(e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-60801, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (1-60801, default 60801): 13

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
p
Partition number (1-4): 2
First cylinder (14-60801, default 14): 14
Last cylinder or +size or +sizeM or +sizeK (14-60801, default 60801): [ENTER]
Using default value 60801

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

mdadm –add /dev/md1 /dev/sdb1
mdadm: added /dev/sdb1

mdadm –add /dev/md2 /dev/sdb2
mdadm: added /dev/sdb2

cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sdb1[0] sda1[1]
104320 blocks [2/2] [UU]

md2 : active raid1 sdb2[2] sda2[1]
78043648 blocks [2/1] [_U]
[>………………..]  recovery =  1.1% (881536/78043648) finish=27.7min speed=46396K/sec
The RAID 1 mirror hard disk set is rebuilding.  In about 30 minutes the operation should have completed and your system will be fully operational.

RAID1 mirror hard disk set has been successfully rebuilt using the replacement hard disk installed in the system.

It seems hardly worth the effort, but ‘2/1’ versus ‘2/2’ really makes a lot of difference.

‘2/1’ indicates that of the expected 2 devices, only 1 device is present; the RAID 1 mirror hard disk set is running in ‘degraded’ mode. With software RAID you manually have to reconfigure the RAID set once repairs have been completed.  This operation is called ‘syncing’ or ‘rebuilding’ of the RAID set. Hardware raid (= a dedicated RAID controller card installed in a server) allows for hot-swapping of hard disks and performs these operations automatically.