I always found Unison to be very helpful. It has a text based or GUI based interface, and quite a few different options to tweak it to what you want (with a little fiddling). It takes quite a bit of time to do the first sync, but after that it's brilliant. You can make it sync one-way, as you want, but it will pretty much get that automatically. It can also delete from the backup or not as you choose.
You also may find issues with permissions which are supported in the ubuntu file format, but maybe not in the external hard drive (depending whether the external hard drive is going to be used in a windows machine, this may be a good thing), so you'll want to sync without the permissions potentially.
Anyway, the nice thing is that with the tutorial it's relatively straightforward to set it up once, and thereafter it's a GUI interface whenever you want to do it.
Here's some info about it:
And here's the tutorial: