In addition to what Jason has shared (I didn't read the link so this may be in there), I'd check the growth setting on the log file. If it is % growth you are getting exponential growth increments and that last one may be bigger than you need. So if your growth increment is 10% and you have a 475GB log file, the next growth will be 47.5GB putting you over the 500GB drive. I always recommend changing the growth increment to a fixed size, which you need to determine based on the activity in your log. Ideally you size the log once and it never needs to grow.