I am having the following problem with a RAID1 software raid partition on my Ubuntu system (10.04 LTS, 2.6.32-24-server in case it matters).

One of my disks (sdb5) reported I/O errors and was therefore marked faulty in the array. The array was then degraded with one active device. Hence, I replaced the harddisk, cloned the partition table and added all new partitions to my raid arrays. After syncing all partitions ended up fine, having 2 active devices - except one of them. The partition which reported the faulty disk before, however, did not include the new partition as an active device but as a spare disk:

md3 : active raid1 sdb5[2] sda5[1]
  4881344 blocks [2/1] [_U]

A detailed look reveals:

root@server:~# mdadm --detail /dev/md3
[...]
Number   Major   Minor   RaidDevice State
   2       8       21        0      spare rebuilding   /dev/sdb5
   1       8        5        1      active sync   /dev/sda5

So here is the question: How do I tell my raid to turn the spare disk into an active one? And why has it been added as a spare device? Recreating or reassembling the array is not an option, because it is my root partition. And I can not find any hints to that subject in the Software Raid HOWTO.

Any help would be appreciated.