Just ran across this posted thread from early in the year and thought it might be important to add something here.
Time Machine can run into trouble when the imagery files start to add up. It depends on the frequency of the grabs, though. In my case, with the frequency for all 3 channels set to 30 minutes in the Preferences, in about a year Time Machine was barely getting finished with its hourly backup by the end of the hour, leading to the impression that it was running all the time. My guess is that the number of files it had to index and cross reference for 'if modified' was getting too large.
I implemented two solutions for this problem:
Most importantly, I initiated a procedure to manually archived the images for all three channels as they aged, moving the files to an external HD. While WC can't access them there, they are still available for movie creation of past weather events.
Second, I located a program called TimeMachineEditor which allows one to set the Time Machine to backup once a day, in my case at 4:30 am.
http://timesoftware.free.fr/timemachineeditor/
