It's a hibernate issue. If the battery dies to the state that it has to save everything to disk and shut down, you'll come back to find you have to cold boot your machine.
I personally put my Intel X25-M in my normal hard drive spot and put the old hard drive in the OptiBay, despite the many people here who will tell you that's a terrible idea. I've been using laptops for 10 years or more, and have never picked up up and ruined a hard drive. And I've got time machine backups of my data if that happens.