I always thought that when a program had a "file lock" no other program could open the file, but would get a windoze error message about the file being in use. Maybe it does just keep a low-rez quickie in memory for the preview (????). It does seem like it takes a bit of time to render, maybe that's because it's loading the full file (???). Sure hope somebody who knows stumbles across this thread and can fill us in.