I've never used a card with my Sony. 200 + books is way more that I need to have at a time, as I use my laptop as my library. I reload every two to four weeks and it is like being in a book store as I decide what to load on to my reader.
I remember seeing something about ".noscan" in a hack to keep the Reader from always scanning the card.
Try
this thread with
this post for a start in finding something that may help.