I found this Q&A (http://superuser.com/questions/148849/backup-mounted-drive-of-an-image-in-time-machine) and this prompted me to ask the following question:
I have two disk images which are scripted to be mounted on login. These two disk images are always mounted to the same location. These two disk images are encrypted TrueCrypt volumes.
Time Machine (TM) will only back up the disk images the first time they are mounted, but not after that. As I modify documents within the volumes throughout the day, the modified timestamps are adjusted properly. However, TM does not back them up. TM never backs up the mount points which are two folders within my home directory.
Any ideas as to why neither the mount point or the image files are backed up? Do the image files have to be closed (unmounted) after being modified for TM to back them up?