Or, how to not waste your hard drive space when you’ve got dropbox installed on two OSes on the same drive.
The basics? Have your Linux partition set to automount your Windows partition, and point Dropbox at your Dropbox folder on the Windows partition.
The detailed way:
Create the folder /mnt/windows
Append
/dev/sda2 /mnt/windows ntfs-3g defaults 0 0
to your /etc/fstab, where /dev/sda2 is your Windows partition. (If you’re on Vista/7, and you bought your computer from a store, it’s likely sda2. Otherwise, take a quick look in Disk Utility.)
And reboot.
Then, go to your Dropbox Preferences > Advanced, and click ‘Move’ next to the “Dropbox location”. Your Windows partition should be mounted as /mnt/windows, so your Dropbox location should look something like this: /mnt/windows/Users/username/Dropbox.