How did you copy the data from zfsnew to zfsbk? my guess is that you either have a sub-folder or a subfolder with the same name as a dataset, and the folder has 2.2TB of data in it, but you can't see it, because you have a dataset mounted over the same mount point. Of course, I assume that you've looked at the /zfsbk to confirm there aren't any subfolders nor any snapshots. My guess would be /zfsbk/media the folder, not the dataset mountpoint. As a test you could unmount all the zfs datasets and see if there are files in each folder.