Just de-activate your internet connection on each ps3's and you should be perfectly fine.
Just transfer (Don't worry you won't lose your cfw as an internet connection is required to make the ps3 update, and also the fw info is stored on the nor not the hdd. (correct me if I'm wrong.)
There's no risk in data transfering as long as you disable your internet (It's normally no risk even with the internet on, but just turn it off

)