Ask HN: How to sync an external HDD with Dropbox?

Input: - data sitting in a Dropbox account (mostly media) - 2 macbooks that sync with said Dropbox account - external HDD (that at the moment has a slightly older copy of the data)

Output: - automated way of syncing Dropbox changes to the external HDD (2-tier backup better than 1-tier, right?)

I guess the solution involves rsync somehow, but I'd like to know what HN thinks is the best way of doing this, and what people use. Bonus Internet points for easily-portable solutions.