thought might be of use to some if you get stuck...
Actually now it works. The only thing I did differently this time was name the output file modded.big. I then deleted the original datafile.big and moved the modded.big file into the directory, and renamed it to datafile.big.
The first time, when it didn't work, I named the output file datafile.big and saved it directly into the directory. I don't know if this is why it didn't work but, for some reason it didn't work twice, then I remade the file, and it works great now. Thank you by the way.