Read this: http://en.wikipedia.org/wiki/Parallel_ATA
starting with the heading "Multiple Devices on a Cable"
On PATA systems, Master/Slave can be selected by (1) the position on the cable or by (2) the selector switch on most PATA devises. Usually there are 3 switch settings: Slave, Master, Cable Select (CS). When set to CS, the position on the cable will determine the hierarchy of slave or master; the master being the connector on the opposite end from the MB connector, the slave in the middle.