Accessing certain files on external HD without entire backup from External HD
If you made your backup using Time Machine, you can restore files from a specific data by selecting the file and then clicking the restore button.



Enter Time Machine

First we need to enter Time Machine. We can do this in 2 ways:
1. Click the Time Machine icon in the menu bar (it's located in top right corner) and choose "Enter Time Machine"
2. Open Time Machine from the Applications folder.



Restoring a file

Once we've entered the Time Machine, browse for the file(s) that you want to be restored. You can choose a different backup time from the right side of the screen.

When you selected a file, you can press the space button to have a preview of the file.

Once you are sure you have the correct file(s), click the "Restore" button in the bottom of the screen.



Source: Apple Support