Disk I/O

Hello, I have a large database that is commonly used for relatively simple SELECT queries. Some of these can take quite a bit of time (hours... overnight) so I've been looking into it a bit. What seems clear is that mserver5 (Oct2014-SP1) is disk-bound, but one curious thing is that most of the time it is write-bound. Despite doing only SELECT queries (no updates or inserts), the amount of data being written is nearly two times as much as data read. Why is that? Is the server restructuring the database on the fly in order to get better performance in later queries? 3X the disk I/O seems excessive so I wonder if there is some way to tune this, or investigate further to see what is happening? Thanks! Tim