Thank you all, it seems to have finished - I'm rebooting.
Just curious why is the State of md3 "active", while the others are "clean"?
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1]
md0 : active raid1 sda1[0] sdb1[1]
1023936 blocks [2/2] [UU]
md1 : active raid1 sda3[0] sdb3[1]
20479936 blocks [2/2] [UU]
[=================>...] resync = 86.6% (17746816/20479936)
finish=0.3min speed=131514K/sec
md2 : active raid1 sda5[0] sdb5[1]
277728192 blocks [2/2] [UU]
md3 : active raid1 sda6[0] sdb6[1]
185151360 blocks [2/2] [UU]
unused devices: <none>
..........Then after some wait:.......
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1]
md0 : active raid1 sda1[0] sdb1[1]
1023936 blocks [2/2] [UU]
md1 : active raid1 sda3[0] sdb3[1]
20479936 blocks [2/2] [UU]
md2 : active raid1 sda5[0] sdb5[1]
277728192 blocks [2/2] [UU]
md3 : active raid1 sda6[0] sdb6[1]
185151360 blocks [2/2] [UU]
unused devices: <none>
# mdadm -D /dev/md3
/dev/md3:
Version : 00.90
Creation Time : Sat Mar 19 22:53:25 2011
Raid Level : raid1
Array Size : 185151360 (176.57 GiB 189.59 GB)
Used Dev Size : 185151360 (176.57 GiB 189.59 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 3
Persistence : Superblock is persistent
Update Time : Thu Apr 28 21:31:12 2011
State : active
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : 1b3668a3:4b6c5593:3d186b3c:53958f34
Events : 0.39
Number Major Minor RaidDevice State
0 8 6 0 active sync /dev/sda6
1 8 22 1 active sync /dev/sdb6
# mdadm -D /dev/md1
/dev/md1:
Version : 00.90
Creation Time : Sat Mar 19 22:52:20 2011
Raid Level : raid1
Array Size : 20479936 (19.53 GiB 20.97 GB)
Used Dev Size : 20479936 (19.53 GiB 20.97 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 1
Persistence : Superblock is persistent
Update Time : Thu Apr 28 21:33:56 2011
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : 8812725b:ea156bc6:3d186b3c:53958f34
Events : 0.48
Number Major Minor RaidDevice State
0 8 3 0 active sync /dev/sda3
1 8 19 1 active sync /dev/sdb3
# mdadm -D /dev/md0
/dev/md0:
Version : 00.90
Creation Time : Sat Mar 19 22:52:12 2011
Raid Level : raid1
Array Size : 1023936 (1000.11 MiB 1048.51 MB)
Used Dev Size : 1023936 (1000.11 MiB 1048.51 MB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Thu Apr 28 21:06:24 2011
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : 87db17c2:d806a38c:3d186b3c:53958f34
Events : 0.14
Number Major Minor RaidDevice State
0 8 1 0 active sync /dev/sda1
1 8 17 1 active sync /dev/sdb1
# mdadm -D /dev/md2
/dev/md2:
Version : 00.90
Creation Time : Sat Mar 19 22:52:32 2011
Raid Level : raid1
Array Size : 277728192 (264.86 GiB 284.39 GB)
Used Dev Size : 277728192 (264.86 GiB 284.39 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 2
Persistence : Superblock is persistent
Update Time : Thu Apr 28 21:17:54 2011
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
UUID : 2db0174b:e45768d5:3d186b3c:53958f34
Events : 0.14
Number Major Minor RaidDevice State
0 8 5 0 active sync /dev/sda5
1 8 21 1 active sync /dev/sdb5