Backing up nas drive to wd my book drive

I have an iMac with a 250 GB internal hd that has become full. I currently backup to a wd 320GB my book using time machine. I bought a my book live nas drive with the hopes of expanding my storage capacity for photos and music which I would now like to store on the nas drive. I also want to backup my internal drive to the nas using time machine. However, since my only copies of the photos and music would be on the nas drive I want to backup these files to my USB my book from the nas drive. Can’t seem to find any information on how to do this. Amy help would be appreciated. Thanks

Try creating a SafePoint there’s info on the manual

thanks for the information.  My problem is that I cannot make a Safepoint on the My Book external drive because it is connected via Firewire to my computer and is not discoverable on the network by the nas drive.  Is there any way to make it such that the My Book is discoverable by the My Book Live drive?