As per the title of the thread, I want to compare the contents of two or more external hard-drives. What I want to do is use these drives as back-ups using Apple's Time Machine. I know that some of the data on one or both of the drives needs to be uploaded to my Mac or another drive first, but all the photos I have on there should be on the Mac already so I don't just want to upload it all. Is there a utility (Mac or PC) with which I can compare the contents of the two drives to spot whether the is data on either one which isn't on the other? Also, has anyone got any tips for using Time Machine as a regular back-up program?