> I have a host with three 120G internal disks and have a RAID 1 mirror
> set up which autoconfigures at boot to be the root device.
>
> I've added the third disk as a hot spare but I find that this
> information isn't retained if the machine is rebooted for any reason and
> I have to manually add it again.

set it up as 3-rd copy. in case of failure - still 2 copies. and one more 
disk to satisfy high read I/O load