I have a dropbox folder on an external hard drive, perfectly synced, on a windows computer. I will use this HD on another computer with windows. I connected the hard drive to the new computer, downloaded the latest version of the agent and configured it to point to the existing folder. The app doesn't allow to use of the existing folder, it offers to rename the existing folder and download everything again or close the app. I did a workaround for the app to accept the existing folder.