Sliding-glass doors generally are far less secure than traditional doors. They can be lifted right off the track if you don't take measures to prevent it. Blocking the track with a dowel stick is an effective measure when the track is inside the building, but the problem of securing the door is not so easily solved when the track is on the outside. Making an outside-track sliding-glass door secure requires some diligence.
Keep the door closed when no one is present to monitor it. Sliding glass doors often are left open in warmer weather, but a screen can be bypassed very quickly. The best way to keep the door secured is to keep it closed and locked.
Replace the latch on the sliding door with an actual lock. Most sliding doors operate with a hook latch that can be pulled open when sufficient force is applied. Replacing that latch with a traditional lock will require some work on both the door and the frame, but your door will be much more durable as a result.
Place a dowel stick between the inside door handle and the frame. The dowel should be long enough to wedge from the handle to the frame when the door is closed. If the door's track were on the inside, the stick could drop into the track; with the track on the outside, the stick needs to be higher so it can prevent the handle from moving.
Drill self-tapping screws into the upper track of the door. They need to allow the door to slide freely back and forth over them, yet hang low enough to prevent the door from being lifted out of the frame.
Place a house alarm sticker on the sliding glass, positioned outward so anyone outside can see it. Even if it's a bluff, the mention of an alarm will make an intruder less likely to attempt entry.
Keeping the area around the sliding glass well-lit at night also will act as a deterrent to an intruder.