I just bought an external hard drive and am using it to back-up my MacBook Pro with Time Machine. Both my MBP hard drive and external hard drive are 500 GB, but I noticed that my MBP HD has 85 GB of files on it while my back-up HD only has about 70 GB. Can anyone tell me why this is the case? I just want to make sure all of my files are backed up properly. Thanks!