Dropbox is not a backup solution. It replicates whatever folder you choose as your Dropbox folder to your computer so basically you are duplicating your external drive on your internal hard drive. That is why it is now full.
yep. Its there.
I am in the process of backing up the internal drive and external drive to BackBlaze. If I get rid of files that I have stored on DropBox that should alleviate problems with the start up drive being too full? Is the start up drive the same drive as the Macintosh HD??