I've found yosemite behaves a bit differently to the previous OS X versions. I do a straight backup of my home folder to an external hard drive every month or so, have done for the past few years without a problem. It's handy if I need to read files off to my windows PC too. Yosemite has some issue with copying the movies, music or photo folders that never happened before, it complains the big files are "in use". I know they are not real big files, but that shouldn't matter for copying. I also have some mini copies of small linux systems we use for work. Originally they were tar'd, but I untar'd them to have easy access to what files should be where and what permissions and owners they have. Mavericks and previous version had no issue at all copying the folder. Now Yosemite says it can't do it. Most folders fail on a bulk copy due to the file cannot be be read or written. If I do copy individual file over then it works fine, it's only on a bulk copy of all folders. So what is different about how yosemite copies files on to a usb external hard drive?