What is on the secondary IDE channel? If there is a free space there, make the new hdd the master. If the primary and secondary IDE are in use, then move one of the optical drives to slave on the primary and add the new hdd to the secondary.
IDE 1- hdd master / optical drive slave
IDE 2 - (new) hdd master / optical drive slave
My thinking is that it is better to have the two hdd on different drives.