Where does Tivo save shows to first ? External or Internal HD ?

Hi guys. First post, go easy :)
I am getting a Roamio today, upgrading from a Q with an external HD.
I want to save a lot of my shows to my external extended myDVR hard drive and just move that from the Q to the Roamio. Does anyone know how I can do that ?
thanks in advance. This is thru my cable company , RCN.