I have been doing time machine backups to a usb hard drive directly connected to my macbook pro. Today I bought a time capsule that I intend use for local backups. Can I occasionally change the time machine settings to use the USB drive (once a month maybe) connected to the MBP, and then go back to the time capsule for daily use? I know they won't be in sync or anything, it just gives me another copy on another drive. When I switch, will it backup just the changes relative to that target drive and not start completely over?