Comment 6 for bug 252365

I can confirm the problem (for 9.04) and I have all disks on the same controller (raid5 with 3 + 1 spare). The spare is recognized sometimes, but not always.

Looking at the boot messages, I strongly suspect that it depends on the sequence in which the disks become available to mdadm. Disks are /dev/sd?5 with ?=[abcd], spare is /dev/sdd5. If the sequence is "bind<sd?5>" with ?=[abc] then the spare is not recognized. If one or more of the [abc] binds show up after "bind<sdd5>", then the spare is recognized.

Seems like spares are not added after the raid has been set active.