Thank you for posting. Am I correct in understanding that you simply want a link in your Brain that points to a directory in your dropbox or to an external hard drive? If you are linking a local file, Windows has a 260 character path limit. This may not effect you on a macOS device, but TheBrain syncs to TheBrain Cloud which is hosted on Microsoft Azure servers. A long file path could/would cause problems, so we suggest sticking with file names that are no longer than 150 characters. (The remaining 110 characters are used by the path leading up to the file name.)
So you do have some options. First, you since you are only creating a link - not moving the files into your Brain, you can always keep track of the full path in your notes. Additionally, Dropbox typically has a URL path for files and directories. You should be able to link the URL to any of these items to a Thought in your Brain.
Finally, TheBrain's log files are typically located in:
Macintosh HD/Users/[user name]/.TheBrain/v9/Logs
The .TheBrain folder is not normally visible on Mac OS (it's a hidden/protected directory). To get to the log folder you can click on Help -> Open Log Directory in TheBrain application, or open Finder: Go -> Go to Folder and type "~/.TheBrain" (without quotes). Then go to: v9 -> Logs.