degraded raid 1 help

[Bazooka Joe]

I have 2 sata 300 gb hard drives mirrored - all partitions are mirrored.

this weekend the home (md6) partition lost one of its members (sda8) but sda
did not fail only one partition.

So I assume i need to hot add back in sda8 to md6

and from the mdadm man page i got

# /sbin/mdadm /dev/md6 -a /dev/sda8
mdadm: Cannot open /dev/sda8: Device or resource busy

am I doing something wrong???  This is the first time i have used mdadm to
manage raid.

do i have to unmount md6??

-chris

extra info below

# /sbin/mdadm -D /dev/md6
/dev/md6:
        Version : 00.90.01
  Creation Time : Sat Aug 27 21:16:47 2005
     Raid Level : raid1
     Array Size : 228420096 (217.84 GiB 233.90 GB)
    Device Size : 228420096 (217.84 GiB 233.90 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 6
    Persistence : Superblock is persistent

    Update Time : Mon Jan 23 16:10:35 2006
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           UUID : dd8c6a34:5c54af22:1c287ca5:c8cf4219
         Events : 0.3277181

    Number   Major   Minor   RaidDevice State
       0       0        0        -      removed
       1       8       24        1      active sync   /dev/sdb8

       2       8        8        -      faulty   /dev/sda8


--------------------------------------
messages log

Jan 22 13:17:21 agwebinc kernel: ata1: command 0x25 timeout, stat 0x51
host_stat 0x21
Jan 22 13:17:21 agwebinc kernel: ata1: status=0x51 { DriveReady SeekComplete
Error }
Jan 22 13:17:21 agwebinc kernel: ata1: error=0x04 { DriveStatusError }
Jan 22 13:17:21 agwebinc kernel: SCSI error : <0 0 0 0> return code =
0x8000002
Jan 22 13:17:21 agwebinc kernel: sda: Current: sense key: Aborted Command
Jan 22 13:17:21 agwebinc kernel:     Additional sense: No additional sense
information
Jan 22 13:17:21 agwebinc kernel: end_request: I/O error, dev sda, sector
159199891
Jan 22 13:17:21 agwebinc kernel: raid1: Disk failure on sda8, disabling
device.
Jan 22 13:17:21 agwebinc kernel:        Operation continuing on 1 devices
Jan 22 13:17:21 agwebinc kernel: raid1: sda8: rescheduling sector 29972968
Jan 22 13:17:21 agwebinc kernel: raid1: sdb8: redirecting sector 29972968 to
another mirror
Jan 22 13:17:21 agwebinc kernel: RAID1 conf printout:
Jan 22 13:17:21 agwebinc kernel:  --- wd:1 rd:2
Jan 22 13:17:21 agwebinc kernel:  disk 0, wo:1, o:0, dev:sda8
Jan 22 13:17:21 agwebinc kernel:  disk 1, wo:0, o:1, dev:sdb8
Jan 22 13:17:21 agwebinc kernel: RAID1 conf printout:
Jan 22 13:17:21 agwebinc kernel:  --- wd:1 rd:2
Jan 22 13:17:21 agwebinc kernel:  disk 1, wo:0, o:1, dev:sdb8
---------------------------------------
mdstat

[root at agwebinc usbdisk]# cat /proc/mdstat
Personalities : [raid1]
md5 : active raid1 sdb1[1] sda1[0]
      104320 blocks [2/2] [UU]

md1 : active raid1 sdb3[1] sda3[0]
      15358016 blocks [2/2] [UU]

md2 : active raid1 sdb5[1] sda5[0]
      15358016 blocks [2/2] [UU]

md3 : active raid1 sdb6[1] sda6[0]
      8193024 blocks [2/2] [UU]

md4 : active raid1 sdb7[1] sda7[0]
      5116544 blocks [2/2] [UU]

md6 : active raid1 sdb8[1] sda8[2](F)
      228420096 blocks [2/1] [_U]

md0 : active raid1 sdb2[1] sda2[0]
      20482752 blocks [2/2] [UU]

unused devices: <none>

-------------------------------------------------------

# /sbin/mdadm --examine /dev/sda8
/dev/sda8:
          Magic : a92b4efc
        Version : 00.90.01
           UUID : dd8c6a34:5c54af22:1c287ca5:c8cf4219
  Creation Time : Sat Aug 27 21:16:47 2005
     Raid Level : raid1
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 6

    Update Time : Sun Jan 22 13:16:52 2006
          State : active
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0
       Checksum : 5cb7ffc9 - correct
         Events : 0.3226885


      Number   Major   Minor   RaidDevice State
this     0       8        8        0      active sync   /dev/sda8

   0     0       8        8        0      active sync   /dev/sda8
   1     1       8       24        1      active sync   /dev/sdb8

-------------------------------------------------------
# /sbin/mdadm --examine /dev/sdb8
/dev/sdb8:
          Magic : a92b4efc
        Version : 00.90.01
           UUID : dd8c6a34:5c54af22:1c287ca5:c8cf4219
  Creation Time : Sat Aug 27 21:16:47 2005
     Raid Level : raid1
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 6

    Update Time : Mon Jan 23 16:27:14 2006
          State : clean
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0
       Checksum : 5cec4a0d - correct
         Events : 0.3277961


      Number   Major   Minor   RaidDevice State
this     1       8       24        1      active sync   /dev/sdb8

   0     0       0        0        0      removed
   1     1       8       24        1      active sync   /dev/sdb8
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