On your motherboard you will have several SATA ports to connect your sata cable to. If you have - say 4, they will be designated Port 1, 2, 3, 4 Picture ClickHere Each sata drive will have it's own separate cable from drive to motherboard. The ports will be marked (very tiny writing) on the motherboard. Your present sata drive that you boot from will be plugged into port 1, which in Windows Computer Management, Disk Management will be shown as 'Disk 0' (don't ask why it's not 1) plug your slave sata into port 2 on motherboard and it will show in Disk Management as Disk 1 (ours is not to reason why...)
The Bios looks to find the first Sata drive that has a bootable op sys on it and boots into that.
For example on mine tonight I have got my W7 harddrive plugged into motherboard sata port 2 and my XP harddrive plugged into motherboard sata port 1. My pc boots up into XP.
When I swap cables round pc boots up into W7.
(I have mine set this way as don't want a dual boot setup, I wanted to keep them entirely separate but be able to access files from both harddrives no matter which I've booted from)