How does kernel decide that a drive is "spare"?

[Dave W <dave+gmane@wuertele.com>]

I have a fileserver with a RAID6 on five 2TB drives.  It appeared to be working
fine before I rebooted it, but now it is complaining

mdadm: /dev/md0 assembled from 2 drives and 3 spares - not enough to start the \
array.

That's strange... I never configured any spare drives.  There should be five
active drives in this array.  I checked /proc/mdstat:

[root@fileserver ~]# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : inactive sdd1[2](S) sdb1[7](S) sdc1[6](S) sdf1[5](S) sde1[3](S)
      9767559680 blocks

[root@fileserver ~]# mdadm -A /dev/md0 -fv /dev/sd[bcdef]1
mdadm: looking for devices for /dev/md0
mdadm: /dev/sdb1 is identified as a member of /dev/md0, slot 7.
mdadm: /dev/sdc1 is identified as a member of /dev/md0, slot 6.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot 2.
mdadm: /dev/sde1 is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdf1 is identified as a member of /dev/md0, slot 5.
mdadm: no uptodate device for slot 0 of /dev/md0
mdadm: no uptodate device for slot 1 of /dev/md0
mdadm: added /dev/sde1 to /dev/md0 as 3
mdadm: no uptodate device for slot 4 of /dev/md0
mdadm: added /dev/sdf1 to /dev/md0 as 5
mdadm: added /dev/sdc1 to /dev/md0 as 6
mdadm: added /dev/sdb1 to /dev/md0 as 7
mdadm: added /dev/sdd1 to /dev/md0 as 2
mdadm: /dev/md0 assembled from 2 drives and 3 spares - not enough to start the \
array.

Why are three drives assumed to be spares?  Is there a way for me to recover
this array?

Thanks,
Dave