Re-add a drive that was removed from a RAID6 array

Hi all,
I need help. I'm a newbie for RAID management.
I have created a RAID6 array with 5 disks. Today, I inadvertently removed the power to one disk and mdadm decided to remove that drive from the array.
Now I have a degraded RAID6 array with 4 disks.
The removed disk is ok, so I want to re-add it to the array.
I can't use the recover function of the OMV web UI, as I already explained in the last comment of issue http://bugtracker.openmediavault.org/view.php?id=672 (where we talked about another issue that caused disks not to appear in the list of available devices to create/extend/recover a RAID array).

I did a search on the net and found that --re-add parameter to mdadm should do the trick. However, when I try to issue the following command:

mdmadm /dev/md127 --re-add /dev/sde

(/dev/sde is the device that was removed from the RAID array)
I get the following error message: "mdadm: Cannot open /dev/sde: Device or resource busy"

Why does this happen? /dev/sde isn't mounted (obviously). Moreover, it's not in the RAID array any more, so I would be surprised to know that it's still locked by mdadm.
Do I have to unmount /dev/md127? Do I have to stop the array before re-adding the missing drive?

Thanks in advance for any help.

Found solution by myself. If anyone encounters a problem like this, here is the solution. I first did:
(please note that after reboot, the /dev/sde that was missing on my RAID6 array had become /dev/sdb)

root@openmediavault:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md126 : inactive sdb[3](S)
      58614680 blocks super 1.2


md127 : active raid6 sdd[0] sde[4] sdc[2] sdf[1]
      175842816 blocks super 1.2 level 6, 512k chunk, algorithm 2 [5/4] [UUU_U]


unused devices: <none>

As you can see, this revealed that the /dev/sdb (once /dev/sde) device was added to another inactive Raid array called /dev/md126. So the solution was this:

root@openmediavault:~# mdadm --stop /dev/md126
mdadm: stopped /dev/md126
root@openmediavault:~# mdadm /dev/md127 --re-add /dev/sdb
mdadm: re-added /dev/sdb

Now /dev/md127 is recovering:

root@openmediavault:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md127 : active raid6 sdb[3] sdd[0] sde[4] sdc[2] sdf[1]
      175842816 blocks super 1.2 level 6, 512k chunk, algorithm 2 [5/4] [UUU_U]
      [=>...................]  recovery =  6.9% (4086448/58614272) finish=65.4min speed=13875K/sec


unused devices: <none>

Please note that the web UI of OMV didn't say anything about /dev/md126: it listed only /dev/md127. This is why I didn't realize what was happening before today.