OK, I think I have a solution but I see a flaw in it I hope someone knows how to fix.
My boot drive on the core where mythbackend is smaller than the second drive I added for user data. If I stop mythbackend, move the contents of /public/data/pvr to my second drive (say /mnt/device/144/pvr) and then put in a symlink from /public/data/pvr to /mnt/device/144/pvr and restart mythbacked then myth should start storing on the second larger drive. But then the process that populates the list of videos on media will find the myth files as well and try to add them to the list of videos, won't it? Any way around this or another way to move the pvr directory to another drive?