Your external hard drive won’t perform any particular action on its own as it is governed by your computer. You can drag and drop / copy and paste files into it from most folders in your system in order to have a secondary copy. You can also link your hard drive to Apple Time Machine:
http://www.apple.com/support/timemachine/
Once a Time Machine backup is completed, you will see an encrypted Time Machine sparsebundle file that can only be (Normally) accessed through Time Machine in case of an emergency.
Regards,
