Until not long ago, Linux was not able to boot from partitions under RAID. That meant that /boot partition can not be mirrored, but the rest of the data can. If that is how system was set-up, then all of the data was mirrored but there is no GRUB/boot setup on the spare HDD. You would need to copy data from /boot partition to "/boot2" (/boot partition on spare HDD) and install GRUB on spare HDD MBR. This can all be done from working system, no need to unplug anything.
You...
