I am Sorry but this question came across my mind when I was asking a computer question in Super User site.
Previosuly, I was writing this
The person who will build the PC for me asked me some question about something like faster Hard Disk, I don't know what exactly he meant, but I have written down what he said, it is called SSD.
I did not jot down important notes during the conversation between the man and I, but later on, I tried to remember what he said and then I wrote down on a paper. Both the conversation and the action that I wrote the notes are already past, so I think I should use past tense and write it this way,
"I don't know what exactly he meant, but I had written down what he said"
But, I doubt if it is grammatically correct and if it can happen in reality, because it sounds like my writing action had finsihed before our conversation began (unless I had some kind of magical power to accurately predict someone's speech)