Its something to do with how the system runs once its enabled. I think that it can lead to instability, but it will not necessarly do so.
I have always had fast writes enabled on my system by default, and I am not sure how to turn it off... never noticed any obvious instability (had this setup for over a year now)
But the point is it will not give you that much increase in performance at all, so if you don't need it do not bother !
Tim
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