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#41
The external harddrive backs up nothing. It is just another disk to which you can backup files or folders using backup programs, copy/paste, drag and drop or whatever method. Best though is to use an imaging program that will image the whole system, all installed programs, all settings, all data and will allow you to restore everything to exactly the state it was in when the image was taken.
There are many imaging programs awailable although I prefer free Macrium. After having tried and experimenting with many others I found Macrium to be the easiest to use, the fastest and it very reliable which is important. What good is a backup if it does not work when you need it.