Via @macdailynews un artículo de Macworld. Sierra se ha llevado la librería libdvdcss que venía por defecto en anteriores versiones del sistema.

Este tutorial (en inglés) te enseña a instalarla usando el terminal para poder recuperar la funcionalidad de Handbrake, el aclamado software para copia de DVDs.
You may buy and rent digital movies, or even get digital copies of your films when you buy DVDs so you can watch them easily on an Apple TV or iOS device. But not all movies offer digital copies, and you may not want to buy movies from the iTunes Store; you may want to own hard copies of your favorite films. Or, you may, like me, buy DVDs of concerts and operas, or have a collection of older DVDs, such as my box sets of The Honeymoonersand The Twilight Zone.

Fortunately, it’s easy to rip DVDs and add them to your iTunes library so you can use them more freely and watch them when you want. Here’s how.

How to rip DVDs with handbrake with macOS Sierra