I recently had a hard drive crash in mac book pro. Having replaced the hard drive my back up disk also decided to crash but I did not worry too much as I had a separate time machine back up. I retrieved most of my files but then realised that loads of my photos were missing. I went back into Time Machine only to discover that my back ups now only go back a week and all the old data has simply disappeared. I have tried pressing the alt key while accessing the Time Machine on the menu bar but there are no other back up volumes. Has Time Machine simply over written all my old data and rendered it irretrievable?