For Windows, plug in the external hard drive and click Yes to the prompt that appears asking if the drive is to be used for backup. On a Mac, go to System Preferences then Time Machine and select your external drive as your backup.
Windows and Mac users both enjoy the convenience of automatic backup creation once the drive has been established as the backup. To retrieve lost data on a Mac, just click on the Time Machine icon in your menu bar and hit Enter Time Machine. From there, you can search through your old backups for the files you lost and restore them to your hard drive. For a Windows PC, you can just go to the Start Menu, type in "backup," and go back to Backup and Restore. Select the Restore My Files or Restore Users Files commands to get those files back.