In my opinion, the best approach to theft prevention is to use a cheap motorcycle cover (no brand name), and use a small lock under the bike to keep the cover secure. Out of sight, out of mind. A chain wrapped around the bike's frame and secured to a nearby immovable object, or wrapped around the frame and rear wheel, will secure the bike from a casual joy rider, but nothing you can carry will stop a professional thief. I do not recommend a disc lock as they are too easy to forget (or in the case of a joy rider, not see) and if you try to ride off without removing it, you and your bike will fall over and the caliper may be damaged ($$$).