[root@muon ~]# mdadm -C /dev/md0 --level=1 --raid-devices=2 /dev/hdb1 /dev/hdb2
mdadm: array /dev/md0 started.
[root@muon ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 hdb2[1] hdb1[0]
      98176 blocks [2/2] [UU]

unused devices: <none>

[root@muon ~]# mdadm /dev/md0 -f /dev/hdb1
mdadm: set /dev/hdb1 faulty in /dev/md0
[root@muon ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Fri Feb  8 16:23:51 2008
     Raid Level : raid1
     Array Size : 98176 (95.89 MiB 100.53 MB)
  Used Dev Size : 98176 (95.89 MiB 100.53 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Fri Feb  8 16:25:34 2008
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           UUID : a806227c:b627f831:c4e4b40b:606317c1
         Events : 0.6

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       3       66        1      active sync   /dev/hdb2

       2       3       65        -      faulty spare   /dev/hdb1
[root@muon ~]# mdadm /dev/md0 -r /dev/hdb1
mdadm: hot removed /dev/hdb1
[root@muon ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Fri Feb  8 16:23:51 2008
     Raid Level : raid1
     Array Size : 98176 (95.89 MiB 100.53 MB)
  Used Dev Size : 98176 (95.89 MiB 100.53 MB)
   Raid Devices : 2
  Total Devices : 1
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Fri Feb  8 16:25:47 2008
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           UUID : a806227c:b627f831:c4e4b40b:606317c1
         Events : 0.10

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       3       66        1      active sync   /dev/hdb2