Here's what I do. In the saved games folder I take my league.lg folder, and copy it to a .zip folder. Then cut/paste the .zip folder over to the external hard drive in a folder called "OOTP saved games" or whatever you want to call it. If you want to update the backup at the end of your playing day, copy your .lg folder to a .zip folder. Cut/paste it to the "OOTP saved games" folder. I think it asks you if you want to overwrite it, and you say yes, and then the league is up to date on the external drive.
When you need to use the .zip folder of your game on your external drive, you can change the name of the game, that you (I assume) need to go back in because something got messed up, to my league (1).lg, and then you won't overwrite it to make sure you don't delete it until you're sure you want to. Then extract the .zip folder from the external drive to your Documents > Out of the Park Developments > OOTP Baseball 20 > saved_games folder, and you're all set up with your game back to the date you want it to be at.
Any questions? Fire away.