Others probably have a better solution than this, but it works for me. I have my photos and Lightroom catalogue all on a 160gb usb powered external drive which travels in my coat pocket, or in my backpack with my laptop depending on circumstances. So when working in Lightroom the little harddrive has to be plugged in (no big deal).
I download the photos to the external drive and backup to the laptop.
I have a second larger external drive which travels in my suitcase and in the evenings I sync the two external drives and delete the laptop backup if I want.
To sync the two external drives I use a program called GoodSync (lots of programs out there will do the same job). I like it because it will sync in both directions, but you have to be real careful if doing this.