What do you want to do with the external?
You have a clone of your machine and you can keep that and set CCC to do scheduled clones of everything, or only selected items if you wish
Maybe doing a full clone once a month or so.
Or now you have a clone and you don't change the system files much you could look at tools like iBackup or LaCie Silverkeeper (both those are free) which offer better options on backing up non system files.simply keeping your external as a mirror of your Mac.
If you want to use an external as an off Mac store why not clear out the USB drive and use that one? If you want to have a clone of the Mac and use the rest of the space on the FW drive for storage (and why not after all) you will probably need to partition it... this is something that really you should have done when you got it because now you will have to make your clone again after the partitioning.
In the end the choice is yours but I think for the short term effort I would make sure the Mac is A1 and then Partition the external drive into 2 (one the size of the Mac HD, and one of whatever is left over... Then you can keep a clone of your Mac and have a storage space... and you could back that up with the USB or use the USB as more storage etc.
Take a look at Shirt Pocket's SuperDuper! it isn't free but it is well worth what they ask and can make "update only" incremental changes to cloned drives which is way way faster...
Hve fun
