Your Dropbox folder must be available when your computer boots andīefore the Dropbox desktop application starts. You may encounter problems if Dropbox occasionally can't access the detached volume holding its folder, however. That said, you can change the location of the Dropbox folder away from its default of ~/Dropbox, so if you have a separate internal or external disk you could put the Dropbox folder there. As far as I'm aware, you can't mount your Dropbox account as some sort of remote file share - its synced files are always written to your local disk.
Update: See nassimhddd's answer below about ExpanDrive allowing you to mount your DropBox account as (external) drive - since
