Zeus:/# mdadm --fail /dev/md0 /dev/hdc1
mdadm: set /dev/hdc1 faulty in /dev/md0
Zeus:/# mdadm --remove /dev/md0 /dev/hdc1
mdadm: hot remove failed for /dev/hdc1: Device or resource busy
Zeus:/# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
md0 : active raid1 hdc1[1] hda6[2]
      20008832 blocks [2/1] [_U]
        resync=DELAYED

md1 : active raid1 hda7[0] hdc2[1]
      57641152 blocks [2/2] [UU]
      [==================>..]  resync = 93.9% (54148736/57641152) finish=0.0min speed=1502551K/sec

unused devices: <none>
Zeus:/#