How to rotate screens separately? Is there a way to rotate an external screen separately and keep the orientation of the inbuilt display fixed?

Using OS X 10.8.5
Yes, you should be able to orient each screen separately. In fact, here is a link of that very thing being done.

To access the screen rotation option in OS X:


Launch System Preferences.
Hold down ⌘ cmd + ⌥ alt/option and click Displays.

On the right side of the Display Preferences, look for the Rotation drop down menu.
Set the rotation you want.

Confirm the change and quit System Preferences.