That's not really how undefined behavior works though. In general, undefined behavior is used to optimize code by assuming it never happens and thus ignore that

That's not really how undefined behavior works though. In general, undefined behavior is used to optimize code by assuming it never happens and thus ignore that case. "implementing" undefined behavior to format the hard drive would mean that compilers would have to implement checks for every possible type of undefined behavior whenever you do something that has a undefined case (Ex. Every time you dereference a pointer). The slow-down would be fairly noticeable.