If I really need to retain something or I am struggling to adequately comprehend it I absolutely have to write about what I am reading. I think you put it correctly that you write "about" what you read not just write what you read. (Not sure if that's what you mean but I like the distinction anyway). My notes aren't really just spewing out what I read, instead I try to condense key points or whatever into my own words, or condense a large amount of material into a short synopsis that I feel accurately captures what I just read. Just reading and writing isn't enough, the "thinking about it and putting it in your own words" part is what does the trick, for me at least. This usually happens in the margins of what I am reading so I don't waste much time.