Thanks I will look at rdiff. I am not sure if rsync is able to “see inside” the *.img files to discern the individual files. If it can then it would be helpful because I could just re-write the same file over and over again and keep backups using rsync or any of the various rsync-derrived tools?
The filesystem will be cold at time of back up because I will need to shut it down, remove the card from the console and put it into my computer’s reader so no worries about that.