ubuntu - re-add to RAID1 is not possible

On Kubuntu 13.10 I have two hdd:
sdb 1TB with two partitions - one in RAID1 and second is simple. Second hdd sda 500GB is whole in RAID1.
Once I unpluged second hdd, put there some other hdd instead, copy somthing from it to sdb and pluged sda back - I know it wasnt great idea, but I was in hurry :)

Now I want to put sdb back to RAID
for

~# mdadm --detail /dev/md/0

i have

/dev/md/0:
Version : 1.2
Creation Time : Wed Oct 31 19:03:13 2012
Raid Level : raid1
Array Size : 11709312 (11.17 GiB 11.99 GB)
Used Dev Size : 11709312 (11.17 GiB 11.99 GB)
Raid Devices : 2
Total Devices : 1
Persistence : Superblock is persistent
Update Time : Thu Dec 12 18:25:33 2013
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0
Name : zizalak:0 (local to host zizalak)
UUID : e7642026:bc8830b1:683d0612:73062736
Events : 143789

Number   Major   Minor   RaidDevice State
   0       0        0        0      removed
   1       8       23        1      active sync   /dev/sdb7

so I tried

mdadm --re-add /dev/md/0 /dev/sda2

but it wrote

mdadm: --re-add for /dev/sda2 to /dev/md/0 is not possible

thanks for any advice
________

dmesg

[ 2.120271] ata2: SATA link down (SStatus 0 SControl 300)

[ 2.124245] ata3: SATA link down (SStatus 0 SControl 300)

[ 2.292377] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)

[ 2.293483] ata1.00: ATA-8: WDC WD5000AAKS-00V1A0, 05.01D05, max UDMA/133

[ 2.293487] ata1.00: 976773168 sectors, multi 16: LBA48 NCQ (depth 31/32), AA

[ 2.294617] ata1.00: configured for UDMA/133

[ 2.294825] scsi 0:0:0:0: Direct-Access ATA WDC WD5000AAKS-0 05.0 PQ: 0
ANSI: 5

[ 2.294972] sd 0:0:0:0: [sda] 976773168 512-byte logical blocks: (500 GB/465 GiB)

[ 2.295003] sd 0:0:0:0: [sda] Write Protect is off

[ 2.295006] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00

[ 2.295019] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't
support DPO or FUA

[ 2.295027] sd 0:0:0:0: Attached scsi generic sg0 type 0

[ 2.296377] ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300)

[ 2.298126] ata4.00: ATA-8: WDC WD10EALX-009BA0, 15.01H15, max UDMA/133

[ 2.298130] ata4.00: 1953525168 sectors, multi 16: LBA48 NCQ (depth 31/32), AA

[ 2.299916] ata4.00: configured for UDMA/133

[ 2.300103] scsi 3:0:0:0: Direct-Access ATA WDC WD10EALX-009 15.0 PQ: 0
ANSI: 5

[ 2.300276] sd 3:0:0:0: [sdb] 1953525168 512-byte logical blocks: (1.00 TB/931 GiB)

[ 2.300290] sd 3:0:0:0: Attached scsi generic sg1 type 0

[ 2.300530] sd 3:0:0:0: [sdb] Write Protect is off

[ 2.300533] sd 3:0:0:0: [sdb] Mode Sense: 00 3a 00 00

[ 2.300586] sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't
support DPO or FUA

[ 2.303858] scsi 5:0:0:0: CD-ROM HL-DT-ST DVDRAM GH22NS50 TN02 PQ: 0
ANSI: 5

[ 2.313709] sr0: scsi3-mmc drive: 48x/48x writer dvd-ram cd/rw xa/form2 cdda tray

[ 2.313714] cdrom: Uniform CD-ROM driver Revision: 3.20

[ 2.313874] sr 5:0:0:0: Attached scsi CD-ROM sr0

[ 2.313996] sr 5:0:0:0: Attached scsi generic sg2 type 5

[ 2.331418] sda: sda1 sda2 sda3

[ 2.331757] sd 0:0:0:0: [sda] Attached SCSI disk

[ 2.348399] sdb: sdb2 < sdb5 sdb6 sdb7 sdb8 > sdb3 sdb4

[ 2.349002] sd 3:0:0:0: [sdb] Attached SCSI disk

[ 2.384447] tsc: Refined TSC clocksource calibration: 2800.115 MHz

[ 2.474936] md: bind

[ 2.476572] md: raid1 personality registered for level 1

[ 2.476863] md/raid1:md1: active with 1 out of 2 mirrors

[ 2.476880] md1: detected capacity change from 0 to 485963399168

[ 2.535184] md: bind

[ 2.843939] md1: unknown partition table

[ 3.380755] md: bind

[ 3.381155] md: kicking non-fresh sda2 from array!

[ 3.381164] md: unbind

[ 3.385373] Switched to clocksource tsc

[ 3.443969] md: export_rdev(sda2)

[ 3.445108] md/raid1:md0: active with 1 out of 2 mirrors

[ 3.445131] md0: detected capacity change from 0 to 11990335488

[ 3.588503] md0: unknown partition table

[ 4.469228] md: linear personality registered for level -1

[ 4.470889] md: multipath personality registered for level -4

[ 4.472510] md: raid0 personality registered for level 0

[ 4.542183] raid6: mmxx1 2335 MB/s

[ 4.610240] raid6: mmxx2 4366 MB/s

[ 4.678377] raid6: sse1x1 620 MB/s

[ 4.746379] raid6: sse1x2 1183 MB/s

[ 4.814406] raid6: sse2x1 1288 MB/s

[ 4.882477] raid6: sse2x2 2633 MB/s

[ 4.950516] raid6: int32x1 1173 MB/s

[ 5.018603] raid6: int32x2 1166 MB/s

[ 5.086652] raid6: int32x4 1055 MB/s

[ 5.154707] raid6: int32x8 814 MB/s

[ 5.154709] raid6: using algorithm mmxx2 (4366 MB/s)

[ 5.154711] raid6: using intx1 recovery algorithm

[ 5.155003] xor: measuring software checksum speed

[ 5.194707] pIII_sse : 10899.000 MB/sec

[ 5.234740] prefetch64-sse: 11130.000 MB/sec

[ 5.234742] xor: using function: prefetch64-sse (11130.000 MB/sec)

[ 5.234902] async_tx: api initialized (async)

[ 5.235791] md: raid6 personality registered for level 6

[ 5.235794] md: raid5 personality registered for level 5

[ 5.235796] md: raid4 personality registered for level 4

[ 5.240501] md: raid10 personality registered for level 10

[ 5.489939] EXT4-fs (md0): mounted filesystem with ordered data mode. Opts: (null)

[ 15.685299] Adding 1951740k swap on /dev/sda1. Priority:-1 extents:1 across:1951740k FS
[ 15.843045] Adding 1952764k swap on /dev/sdb6. Priority:-2 extents:1 across:1952764k FS