Law School Academic Support Blog posts some of the myths that law school students harbor regarding exam preparation.
I agree with the list for the most part. I would challenge some of the wording of number 5 as it pertains to open book exams that include open notes (not the part about studying as hard...that's correct).

Here is number 5.
- You do not have to study as hard for an open-book exam. Open-book exams are usually a trap. Students who do not learn the
material as they would for a closed-book exam often have to look
everything up. Only a general, surface knowledge of the material is
often the result of believing this myth. And, we all know that you
rarely have spare time in an exam to look much up in your materials.
In my experience the last sentence is wrong if the professor allows open notes for the exam. While I may feel rushed and hurried at first I've only had one exam where I was really rushed. In fact, with a large majority of my exams I have time to go over my exam and spell-check and proofread somewhat. This is one of the huge advantages of using computers for exams.
One thing many professors tell you is to not just dive into writing your answer. Take a few minutes before you start writing to make to organize or make an outline of where you want your answer to go. While agree there is not going to be time to look much up at all in a bulky casebook, if there is time to organize, there is time to look things up in your notes in order to better organize.
And don't get me wrong, it's not that you should spend less time in preparation of on open book/open notes exams in relation to closed, but for me it's a different type of study and preparation.
Open book law school exams are about perfecting an outline for me. Using tabs and a table of contents or index to create an outline of the class in which I can move quickly and find what I need.
And I'm not afraid to go on the longish side on class outline lenghth for open note exams because, while you still need to understand the material, memorization is not as important. You'll find that just the process of making a more detailed outline and structuring it goes a long way to learning the material, itself.
Making an outline like this for open book/open notes exams takes a lot of time in preparation of the exam, but it is a different type of studying than closed book. Closed book is more about making a shorter class outline and (while most law professors hate to admit it) memorizing that outline.
So while I agree that you will have to spend just as much time preparing and studying for closed book exams as for open book with open notes, I think it's wrong to not adjust your preparation approach somewhat for each.