Best way to use QNAP TS-231 NAS and Time Machine

I have just got a QNAP TS-231 with two 3TB drives. My current set up is an aging iMac with 750GB HD, a G-Drive 1TB external drive that I keep my iTunes and Photos stuff on and that I have been using for Time Machine as well. It just struck me that I am only backing up what is on my iMac and not what is on the G-Drive. I would like to move the media and pictures to the NAS so that the rest of our family can use the Home Sharing and we can also use the Sonos. I think that my basic NAS set up uses 1 drive for storage and the other to mirror that. If I move these files to the NAS and am getting the mirrored copy on the second disk (essentially a back up) does it make sense to use the G-Drive for Time Machine to back up what is on the iMac only? Am I missing something?