Ok, stupid question. I want to get an external hard drive for back up. But they're big. I don't have room to leave one on my desk all the time. I know it doesn't need to be plugged in all the time. But everything I've heard about TM says you pick a time for it to back up daily. Can I just program it to back up each time I plug the drive in? Also, if anyone has any recommendations for an affordable, decent, drive, I'm open to them.