How to keep backup of Outlook in your computer ????

Click File>New>Outlook Data File.  Browse to a folder on your external hard drive, give the PST a name (or accept the default), and click OK.  Give the folder set a display name (what you want Outlook to display in the Navigation Pane) - you can also accept the default - and click OK.  You now have a PST on the external drive.  Now, copy any folders you want on that external drive to the root of the folder set on the external drive.

You could instead, if you wish, use a backup program to make automatic copies of your PST to the external hard drive and then only open those folders if you need to recover after a data loss.