When I read this quote I quite liked it.
"There are two things about hard drives, either they are going to fail, or they have failed."
Thinking of it in that way means you won't (or shouldn't) rely on some known % failure rate statistics or thinking my RAID has this low chance of failing so I will be fine etc, as at some point you know they will fail. Enterprise quality or not.
It is all well and good if you have a RAID array where you can suffer several drives failing at the same time and have spares ready to rebuild but have you asked what if another one fails before rebuild? What if they all fail? Ask this because in my and others experience when one thing goes wrong it just so happens it is when you need it most. (I think this is known as Murphy's Law) I've heard stories of someone telling me the chances are so low.. followed by but it just so happened on this one occasion and.. Also recently I suffered several drives fail within one month of one another after about 5-6 years of use (more on that one in another post)