Let me explain what i would like todo.

i have:
mbp c2d
external fw400 enclosure with 200gb hd
internal 120gb hd inside mac

This is what i want to do:

id like to use the fw400 enclosure while at my desk, easy enough. any work i do on this, id like to be mirrored into the internal hd. so when im on the go, everything is the same as when i left my desk, only from external to internal.

now when i get home, id like for my internal to sync with my external, overwriting all OLD files with new. But not rewriting over folders (like osx does), merging them into one.

any thoughts? ive got the free version of SuperDuper, like every mac owner should. But i dont want to have to monitor using it. id like it automatic possible. and i dont obviously wanna rewrite over the entire HD (int or ext) every time they want to sync (depending on the situation)
im assuming superduper will do all this stuff for me, but like others, id like to explore free options before i drop 30$