The current version (1.0.7) of B&N's ereader for BB cannot read epub format books. There is also the problem of DRM, converting a book with no drm from epub to pdb will not stop the ereader from trying to use the B&N DRM scheme. Which of course results in the book not opening.
As to the "unable to sync" error, have you tried rebooting the BB after loading the book? I occasionally get a "media card removed, unable to sync" message after directly adding media to the sd card but rebooting takes care of it.