Can't play DVD on Mac

Hi, I can't play a particular DVD on my Mac. It loads, but only plays the 15-second intro file. All other options are "Not Permitted". I'm playing it using an external drive. There's nothing wrong with the set-up - it plays other DVDs fine - and it's the correct region. Also there's nothing wrong with the DVD itself, as it plays on other devices, ie. laptop. Any ideas? Thanks.