Have you given "Permission" for LR to access that drive?
If not, here's how to do that:
Go to your System Preferences and select Security and Privacy (found in the top section). Now click on the Privacy setting (#2 below). Now click on the lock on the bottom left (#3) and provide your administrator code. Then click on the "+" symbol at #4. This will bring up the standard Mac Open window. Look for Bridge, select it, it will now appear in that window and make sure it's checked. Close everything down and see if that makes it all work.