Hi,
Okay when mastering to the beta tape try dragging the final edit/composition file into the output/preview window. You should get options to master to tape etc. See if this works.
I would also make sure your settings aren’t conflicting blackmagic 8MB, 10MB etc.
I’ve had troubles in Final Cut with the latest driver… as a last resort try installing an older version. I think 5.7 is the version I went back to. I also have a Mac Pro.
Hope this helps.
grafx that flow!