Take the idler pully out or get one with raised edges, some of the clone 450's came with them. In my experience the (smooth) idler pully would let the belt walk off of the drive pully and wad up between the side of the case/pully. It works fine without the idler. I believe Align put somthing out a few years back about this. If you have a picture of the damage, post it if you can. I can't remember having a belt take out a case before, other than me slamming into somthing as a dirrect result of the belt coming apart, (or any of the thousands of other reasons I have found to do so). You say the boom was bent upward, you could be running your tail rotor backwards because the belt should be pulling down on the end of the boom if the rotor was to stop. The tail rotor should spin in the dirrection that if the bladed were to touch the ground, the dirt would fly twards the front of the heli like a wheel in reverse. The fact that the tail has come apart twice in a row has me thinking something like that is what happened. (Don't ask me how I might have learned to look for those kind of things!)