I have been storing my entire Lightroom 4 photo library on a Hard Drive, and it has been working for several years. But recently, I plugging in the Hard Drive and Lightroom is not recognizing it. The Hard Drive works fine and I can view the entire library in Finder on the Mac, so there's nothing wrong with the Hard Drive. It is just that LR is not reading it. The only new thing I did was install Yosemite. I don't know if that could change things in any way? I would really appreciate any help and advice. I have been searching online and haven't found any relevant answers. Thanks!