I've bought a new WD My Passport Essential 500GB a couple of days ago and after reformating it I set about it backing up my data using Time Machine. I left it on overnight to come back to it "Stopping back up...", and left it a few more hours but it still had the same status. I then restarted my iMac and noticed the main start-up drive inside the iMac now has 3.9GB which is way less than it was before I started using Time Machine. Anyone know what this is? It's almost as if it made a replica of itself both on the external and the internal although I don't think there would be enough space on the internal to do a full replica, so I'm puzzled... The back-up seems to have silently corrected itself though as the Time Machine System Preferences lists an earliest and latest back-up!