Script at /etc/init.d does not run at start-up

Solution 1:

It may be a good way if you use insserv instead of sudo update-rc.d dropbox.sh defaults.

sudo insserv

From man insserv:

insserv  is a low level tool used by update-rc.d which enables an installed sys‐
tem init script (`boot script')

If you get the command not found error, you may fix by running the following command:

sudo ln -s /usr/lib/insserv/insserv /sbin/insserv

See insserv -h for help.