I have two Macbooks and two external drives and I need to have everything backed up, so I'm thinking of getting me a time capsule. On the external drives I store music and photos, and I connect them from time to time to either of my Macbooks. How would I have things set up with regards to the external drive so they get backed up when connected to the macs? I would like the external drive backups to be separate from the backups of the macbooks' internal drives. Is that how things work by default?