mdadm --manage /dev/md1 --add /dev/sdb
mdadm: /dev/sdb reports being an active member for /dev/md1, but a --re-add fails.
mdadm: not performing --add as that would convert /dev/sdb in to a spare.
mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sdb" first.

mdadm --detail /dev/md1
/dev/md1:
        Version : 0.90
  Creation Time : Sat May  2 11:10:24 2009
     Raid Level : raid5
     Array Size : 976772992 (931.52 GiB 1000.22 GB)
  Used Dev Size : 488386496 (465.76 GiB 500.11 GB)
   Raid Devices : 3
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent
 
    Update Time : Tue Mar 20 18:45:11 2012
          State : clean, degraded  
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0
 
         Layout : left-symmetric
     Chunk Size : 64K
 
           UUID : 3a6a9669:ac3611d9:399fa505:cd0a86fd
         Events : 0.39165
 
    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       8        0        1      active sync   /dev/sda
       2       8       32        2      active sync   /dev/sdc

root@lanfeust:~# smartctl -a /dev/sdc
smartctl 5.41 2011-06-09 r3365 [x86_64-linux-3.1.0-1-amd64] (local build)
Copyright (C) 2002-11 by Bruce Allen, http://smartmontools.sourceforge.net
 
=== START OF INFORMATION SECTION ===
Model Family:     Seagate Maxtor DiamondMax 21
Device Model:     MAXTOR STM3500630AS
Serial Number:    6QG11J79
Firmware Version: 3.AAE
User Capacity:    500 107 862 016 bytes [500 GB]
Sector Size:      512 bytes logical/physical
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   7
ATA Standard is:  Exact ATA specification draft version not indicated
Local Time is:    Tue Mar 27 18:27:08 2012 CEST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
 
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
See vendor-specific Attribute list for marginal Attributes.
 
General SMART Values:
Offline data collection status:  (0x82) Offline data collection activity
                                        was completed without error.
                                        Auto Offline Data Collection: Enabled.
Self-test execution status:      (   0) The previous self-test routine completed
                                        without error or no self-test has ever  
                                        been run.
Total time to complete Offline  
data collection:                (  430) seconds.
Offline data collection
capabilities:                    (0x5b) SMART execute Offline immediate.
                                        Auto Offline data collection on/off support.
                                        Suspend Offline collection upon new
                                        command.
                                        Offline surface scan supported.
                                        Self-test supported.
                                        No Conveyance Self-test supported.
                                        Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                                        power-saving mode.
                                        Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                                        General Purpose Logging supported.
Short self-test routine  
recommended polling time:        (   1) minutes.
Extended self-test routine
recommended polling time:        ( 163) minutes.
 
SMART Attributes Data Structure revision number: 10
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000f   115   082   006    Pre-fail  Always       -       96998496
  3 Spin_Up_Time            0x0003   094   093   000    Pre-fail  Always       -       0
  4 Start_Stop_Count        0x0032   100   100   020    Old_age   Always       -       1007
  5 Reallocated_Sector_Ct   0x0033   100   100   036    Pre-fail  Always       -       2
  7 Seek_Error_Rate         0x000f   087   060   030    Pre-fail  Always       -       554513637
  9 Power_On_Hours          0x0032   082   082   000    Old_age   Always       -       15939
 10 Spin_Retry_Count        0x0013   100   100   097    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   099   099   020    Old_age   Always       -       1246
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
189 High_Fly_Writes         0x003a   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0022   066   037   045    Old_age   Always   In_the_past 34 (Min/Max 26/34)
194 Temperature_Celsius     0x0022   034   063   000    Old_age   Always       -       34 (0 18 0 0)
195 Hardware_ECC_Recovered  0x001a   096   054   000    Old_age   Always       -       197484494
197 Current_Pending_Sector  0x0012   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0010   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0000   100   253   000    Old_age   Offline      -       0
202 Data_Address_Mark_Errs  0x0032   100   253   000    Old_age   Always       -       0
 
SMART Error Log Version: 1
No Errors Logged
 
SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Short offline       Completed without error       00%     14840         -
# 2  Short offline       Completed without error       00%     14840         -
# 3  Extended offline    Aborted by host               90%     14840         -
# 4  Short offline       Completed without error       00%     14839         -
# 5  Short offline       Completed without error       00%     14838         -
# 6  Extended offline    Aborted by host               90%     14837         -
# 7  Extended offline    Aborted by host               70%     14835         -
# 8  Short offline       Completed without error       00%     14832         -
# 9  Short offline       Completed without error       00%     14829         -
#10  Short offline       Completed without error       00%     14828         -
#11  Extended offline    Aborted by host               90%     14827         -
#12  Short offline       Completed without error       00%     14825         -
#13  Extended offline    Aborted by host               90%     14825         -
#14  Extended offline    Aborted by host               90%     14825         -
#15  Short offline       Completed without error       00%     14823         -
#16  Extended offline    Aborted by host               90%     14823         -
#17  Short offline       Completed without error       00%     14820         -
#18  Short offline       Completed without error       00%     14819         -
#19  Short offline       Completed without error       00%     14818         -
#20  Extended offline    Aborted by host               90%     14817         -
#21  Short offline       Completed without error       00%     14816         -
 
SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

root@lanfeust:~# smartctl -l selftest /dev/sdc
smartctl 5.41 2011-06-09 r3365 [x86_64-linux-3.1.0-1-amd64] (local build)
Copyright (C) 2002-11 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF READ SMART DATA SECTION ===
SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%     15949         -
# 2  Short offline       Completed without error       00%     14840         -
# 3  Short offline       Completed without error       00%     14840         -
# 4  Extended offline    Aborted by host               90%     14840         -
# 5  Short offline       Completed without error       00%     14839         -
# 6  Short offline       Completed without error       00%     14838         -
# 7  Extended offline    Aborted by host               90%     14837         -
# 8  Extended offline    Aborted by host               70%     14835         -
# 9  Short offline       Completed without error       00%     14832         -
#10  Short offline       Completed without error       00%     14829         -
#11  Short offline       Completed without error       00%     14828         -
#12  Extended offline    Aborted by host               90%     14827         -
#13  Short offline       Completed without error       00%     14825         -
#14  Extended offline    Aborted by host               90%     14825         -
#15  Extended offline    Aborted by host               90%     14825         -
#16  Short offline       Completed without error       00%     14823         -
#17  Extended offline    Aborted by host               90%     14823         -
#18  Short offline       Completed without error       00%     14820         -
#19  Short offline       Completed without error       00%     14819         -
#20  Short offline       Completed without error       00%     14818         -
#21  Extended offline    Aborted by host               90%     14817         -