I use a thumb drive to backup my iMac with OSX 10.7.5. What I do is I copy the iTune folder, the iPhoto Library, and a directory called "Important" over to the thumb drive. I would like to do this more often, but it takes a while to copy over the Important directory since it has about 13Gbs of data in it. My question is this, what if I don't copy the entire thing, completely replacing the old file like I have been? If I choose not to replace, will OSX know which files to add? I am just a little worried about trusting it. For example, what if I had a directory inside of the Important directory called "PDFs" (Important/PDFs). Let's say I now deleted that directory on my Mac. When I did the backup and copied the entire Important directory to the thumb drive, would it now delete that PDFs directory?