Jettison is a utility that ejects external disks from your MacBook automatically. It eliminates the hassle of manually ejecting external drives before you put your MacBook to sleep.
Why do I need to eject hard drives before unplugging them? You risk losing data if you don’t. OS X doesn’t write information to the drive immediately - it often caches it to make operations faster. If you unplug a drive without first ejecting it, OS X may not have a chance to actually write that data to the drive!