They are hot swappable. So you can have a 500 gig drive filled with games plus your main storage of 1tb when you out grow that you can buy a 1tb drive and in total have 2.5 tb of storage. Keep certain games on your 500gig and certain ones on your 1tb and the most played games on systems. Some games like COD may end up filling a 500gig drive or close to it at some point so you can simply have a call of duty cart
I am also almost 100% certain that someone will make a drive hub for the system with a wire connected in the back that runs to a small hub allowing you to put a few drives in it and have easy access for it.
I'd also expect a 2TB option over the next year , I think segate/ms are just waiting for the single chip 2TB options to come in
Don't think there is enough room in there for two 1tb chips.
Last thoughts , I wonder when the segate deal ends and we see other companies tackle this. Perhaps this holiday season is the last exclusive block for segate ?