I have certain jpgs on an external hard drive that lightroom will not recognize. Why does this happen? How do I resolve?
They might be seen as sidecar files of RAW files and already recorded in the catalog although not shown. If you don't have the option checked to "Treat JPGs next to RAW as separate files" LR will import them, when you import the RAW files off the camera memory card, place Copy, them onto your drive, in the same folder as the RAW, and record them as imported and stop them from being imported a second time.