If you put it in the garage, definitely bolt it down and definitely use a dehumidifier like a Golden Rod. Then again, I'd recommend both those things even if it was inside. My biggest concern would be that you are placing your safe outside the protection of your alarm system (assuming you have an alarm and it is setup the way most are) and that when your garage is open, people driving by can see you have a safe. You can get around both these things. Try placing other stuff strategically in your garage to make it harder to see from the road. Maybe put it on a side wall and place a stand up freezer next to it on the side closer to the door. Also, see if your alarm system can be added to the garage with either a sensor on the door or maybe a motion detector in there.