Trying to sync external HD, message - "can't sync, access denied"

I have an external hard drive with a folder, “Organized Movies” with sub-folders “2009, 2010…2015” and sub-folders in each of these, “01 January 2015 Movies, 02 February 2015 Movies…” I synced once in May with total success! Now, when I try to sync, I get a Dropbox message, "Can’t sync “Organized Movies” (access denied), and I get a Boxifier message that shows grey folders and no check marks. When I click on Computer / HD / “Organized Movies” / Boxifier - Sync with Dropbox, I get “Organized Movies” in red and the message, “This name is already in use by another folder. Please choose another name”.

I saw a previous forum question that seems very similar to my problem, but I do not understand where to go to find Dropbox preferences.
Thank you!