Raid1 with active and spare partition

Daniel: Initially, double-check to see if the spare is being integrated into the array by doing a:

cat /proc/mdstat

which should report if there is a build process going on, and how long to expect that to take.

If no building is occurring, then try the following

mdadm /dev/md3 --remove /dev/sdb5

mdadm /dev/md3 --add /dev/sdb5

And report back how that works for you. See http://linux.die.net/man/8/mdadm for more details.

Quite late, but doing this would just enable the spare drive:

mdadm --grow /dev/md3 -n 2

As the manpage says:

For create, build, or grow: -n, --raid-devices= Specify the number of active devices in the array. This, plus the number of spare devices (see below) must equal the number of component- devices (including "missing" devices) that are listed on the command line for --create. Setting a value of 1 is probably a mistake and so requires that --force be specified first. A value of 1 will then be allowed for linear, multipath, RAID0 and RAID1. It is never allowed for RAID4, RAID5 or RAID6. This number can only be changed using --grow for RAID1, RAID4, RAID5 and RAID6 arrays, and only on kernels which provide the necessary sup‐ port.