I bought my 1 TB My Passport Essential with the idea that I would store system backup images on it. Then when I needed to restore my system, I would boot in my recovery software and have it load one of my backup images.
I do this now with both an older Passport and an older My Book.
So I asked WD support whether the firmware would interfere with the booting process.
Unfortunately, I am reminded by the forum administrator that I cannot reveal the contents of the email response for privacy reasons. So if you are interested in booting from your drive, I would urge you to contact WD support directly.
I think I can say that the answer that I got was discouraging.