mdadm: hot remove failed for…

My RAID1 is degraded and I need to replace /dev/sdb. However, I could mark /dev/sdb5 (in /dev/md1) as failed and removed it, but I can't do the same with /dev/sdb1 (/dev/md0 which is mounted on /):

root@kes:~# mdadm /dev/md0 --fail /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md0

root@kes:~# mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90
  Creation Time : Wed Apr  7 03:00:39 2010
     Raid Level : raid1
     Array Size : 975185536 (930.01 GiB 998.59 GB)
  Used Dev Size : 975185536 (930.01 GiB 998.59 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Mon Feb 13 12:27:41 2012
          State : active, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           UUID : cdc01e79:774eba08:ade2cb46:d0df0469 (local to host kes)
         Events : 0.67569047

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       2       8       17        1      faulty spare rebuilding   /dev/sdb1

root@kes:~# mdadm /dev/md0 --remove /dev/sdb1
mdadm: hot remove failed for /dev/sdb1: Device or resource busy

So what am I doing wrong?

Try:

echo "idle" > /sys/block/md0/md/sync_action
mdadm /dev/md0 --remove /dev/sdb1

worked for me.