At this current point in time I do not have a back up for my MacBook Pro (I know, shock, horror...don't judge). Anyway, I fully understand it's time to change this fact but I'm unsure as to how to go about it. I have a Retina MacBook Pro with a 256GB SSD and I'm thinking about buying a 500GB (will that be enough?) external hard drive to use as my Time Machine back up. However, I understand that Time Machine works effectively when it is set to back up automatically in which case it can take daily or even hourly back ups of your machine. But, I don't want to have an HDD plugged into my MacBook 24/7 just to allow me this. What would be the best advice on how and when to back up a MacBook Pro with Time Machine? Update: I purchased a 500GB USB 3.0 Toshiba portable hard drive and have set up Time Machine! I've decided I will be performing a back up once a week as this seems to be the most practical of options. Thanks for all your help!