The disks needs power cable from the PSU and data cable from the motherboard. If these are IDE disks on the same cable then the primary needs to have its jumpers set for master and the secondary slave or you can use cable select for both (jumper settings sometimes displayed on drives or available at man website). If the additional drive is going on a separate IDE cable then master slave is unimportant.
SATA drives are one drive per cable/connector and no master slave therefore.