I am not sure if it is wise to backup the root directory, because: a) BT sync AFAIK does not distinguish between a symlink and a regular file/directory, so there can be an infinite recursion. I am not sure if BT sync can handle /proc, /sys and /dev properly. c) Did you consider permissions? d) You hay have connected some other device (e.g. an external HDD or an USB stick), which would be added to the sync. e) Do you really need a backup of /bin, /boot and so on?