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Below are some notes on using Linux Software RAID. It's not complete yet of course :)

Marking a damaged disk as failed:

mdadm --manage --set-faulty /dev/md1 /dev/sdd1

Removing the damaged disk from the array:

mdadm /dev/md1 -r /dev/sdd1

Shutdown your system, bring it up into single user mode and partition and format your new disk. Set the partition type as Linux Software RAID autodetect. Of course you need to partition your new disk the same as the previous one. Bring the machine up into it's default runlevel, either by rebooting or by doing init 5 or init 2 depending on your Linux distribution.

Add your new disk to the array:

mdadm /dev/md1 -a /dev/sdd1

Viewing the status of the array (the one shown here is repairing):

[root@host ~]# cat /proc/mdstat 
Personalities : [raid1] 
md1 : active raid1 sdc1[2] sdd1[1]
      142327744 blocks [2/1] [_U]
      [==========>..........]  recovery = 54.5% (77655168/142327744) finish=65.2min speed=16503K/sec
md0 : active raid1 sdb1[1] sda1[0]
      142327744 blocks [2/2] [UU]
      
unused devices: <none>

Showing status detail for a particular array:

[root@host ~]# mdadm --detail /dev/md1
/dev/md1:
        Version : 00.90.01
  Creation Time : Sat Jan 13 08:24:54 2007
     Raid Level : raid1
     Array Size : 142327744 (135.73 GiB 145.74 GB)
    Device Size : 142327744 (135.73 GiB 145.74 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Wed Aug 27 09:10:48 2008
          State : clean, degraded, recovering
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1


 Rebuild Status : 55% complete

    Number   Major   Minor   RaidDevice State
       0       0        0       -1      removed
       1       8       49        1      active sync   /dev/sdd1
       2       8       33        0      spare   /dev/sdc1
           UUID : cb59fcb5:d61a134b:3e799237:ab0c3b4f
         Events : 0.25582871

View the Detail of a Particular Disk in an Array:

[root@host ~]# mdadm --examine /dev/sdd1
/dev/sdd1:
          Magic : a92b4efc
        Version : 00.90.00
           UUID : cb59fcb5:d61a134b:3e799237:ab0c3b4f
  Creation Time : Sat Jan 13 08:24:54 2007
     Raid Level : raid1
    Device Size : 142327744 (135.73 GiB 145.74 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1

    Update Time : Wed Aug 27 09:20:10 2008
          State : clean
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1
       Checksum : ce0b5011 - correct
         Events : 0.25583249


      Number   Major   Minor   RaidDevice State
this     1       8       49        1      active sync   /dev/sdd1
   0     0       0        0        0      removed
   1     1       8       49        1      active sync   /dev/sdd1
   2     2       8       33        0      spare   /dev/sdc1

• Linux Software RAID Howto
• http://www.networknewz.com/2003/0113.html
• http://acd.ucar.edu/~fredrick/linux/fedoraraid/