A couple of times I have been working on a file, run the first couple of steps then tweek it or add the next step. When adding to a file that is working, it is a good idea to make a copy of the file, then make your changes to a copy.
Forgot to do that yesterday and wound up with a corrupted file. Once you have a copy of a copy of a copy, working the way you want, it is a simple matter to erase the first generations of the file. Wish I did that yesterday, wiped out a couple of very nice boards.
Oh Well, Off to try again this morning.