How to sync folders (or files) that exist outside the "My Dropbox" folder:
This can be achieved using each platform's version of symbolic links.
Windows (Needs NTFS file system)
Use either the JUNCTION utility from Sysinternals, or the MKLINK command built in to Windows Vista and Server 2008. Dropbox will not monitor for file changes in the symlink folders and will update them only at startup. To avoid this - move the desired folder to "My Dropbox" and create a symbolic link from it's old location so that it can still be accessed from there:
1. Move your folder to the "My Dropbox" folder.
move "C:\Path\To\DesiredFolder" "C:\Documents and Settings\User\My Documents\My Dropbox\"
2. Create the symbolic link at the old location.
junction "C:\Path\To\DesiredFolder" "C:\Documents and Settings\User\My Documents\My Dropbox\DesiredFolder"
or
mklink /D "C:\Path\To\DesiredFolder" "C:\Users\Steve\Documents\Dropbox\DesiredFolder"
Or, if you prefer a GUI, install Link Shell Extension. There is a tutorial on how to use Link Shell Extension in conjunction with Dropbox. Instead of replacing the "My Dropbox" folder, drop the link inside My Dropbox.
Use SyncToy to echo changes from another folder to your Dropbox folder. This keeps 2 copies on disk though.
OS X and Linux
Use a Symlink. This, similar to an Alias, is what you can use to link in files or folders from outside of Dropbox.
Use the ln command, for example:
ln -s /path/to/folder/name_desired_folder/ ~/Dropbox/desired-folder
You can also choose to put the original folder inside your Dropbox folder, and then create links in the other direction.
(More information about "ln" go into the Terminal and type: man ln)
This works with files too:
ln -s ~/Dropbox/desired-file /path/to/folder/name_desired_file
However, if you put a symlink in your Dropbox folder that links to a single file, updates to the file sometimes cause the link to be replaced by a copy of the file itself, and the original 'linked to' file is not synchronized anymore. A safe solution seems to be to put only symlinks to folders in your Dropbox folder, or to put the original file into your Dropbox folder and then create symbolic links to your file at the other location(s) you want to access the file from.
Question: How about, in the case of single files, using a hard link instead? Would Dropbox destroy/recreate the file on syncing, hence destroying the hard link?
Answer: I tried to hard link StickiesDatabase on my Mac: ln ~/Dropbox/varmuuskopiolinkit/StickiesDatabase ~/Library/StickiesDatabase the link was destroyed every time I edited my Stickies. I think the problem is with the Stickies, not with Dropbox; when you quit Stickies, it will delete the old StickiesDatabase and immediately create new one. That will break hard link, but not a symlink. Symlinking works fine with Stickies!
Another easy way to do this with Terminal is type the ln -s part, then from Finder drag the Dropbox folder then drag the folder/file that you want into the Terminal window and hit return.
Note that an Alias file or folder does not work.
Warning: Do NOT move your OSX Applications tree into DropBox and symlink it. This is due to a KnownIssue related to the handling of large am mounts of files
OS X Specific: AUTOMATOR
There is a Automator workflow that is available for download. This work flow contains everything needed to create a symlink in just a few clicks. And works within the right click contextual menu.
The Tutorial on symlink automator workflow can be download here:
The workflow files can be downloaded here:
Direct links were not made on files because of some sort of java scripting error in the wiki
(NOTE that the tutorial available for download above contains the instructions for installation of the workflows)
OS X Specific: SymbolicLinker
If you find that venturing into the Terminal is a bit too hard to make a Symlink, there's a great tool that streamlines the process called SymbolicLinker, by Nick Zitzmann. This provides a contextual menu (for Leopard and below) or a software service (for Snow Leopard) that allows the user to create symbolic links to files without leaving the Finder.
Linux using Nautilus
Navigate with Nautilus file manager to the desired folder, right click it, select Make link , then move the resulting Link to desired-folder to the Dropbox folder. If the desired directory is outside your home directory, you may have to start gksudo nautilus via Terminal to make those links.
Known issues
Windows may not update changes to symlinked files automatically.
Changes made to a junction point-linked file/folder on Windows may not update the Dropbox mirrored copies until you close and restart Dropbox. See http://forums.getdropbox.com/topic.php?id=583 for more details. Recommend keeping the real files in the Dropbox folder and linking into other locations rather than the other way around.