I've been accustomed to being able to transfer a day's content from my Macbook to my hard drive, and vice versa, by dragging that file to the "All Photographs" section of the other device. At the moment it bounces back instead of transferring. Maybe I pressed the wrong key on the Edit tab?
Could you please be a bit more specific about the role Lightroom plays within your issue and post the exact wording of the message appearing?
Also, please tell us the version of Lightroom as well as your Mac version.