Cant help you out with the sharing, Im a bit fuzzy in that area too.
But for your other question about internal or external drive, external would make sense if that 2nd TB is for your backups. However, as mentioned, its an added cost for no real gain.
Internal would also make sense for the same reason. However, internals are not suited as backups because of the computer get stolen/damaged there goes your backup too.
But like the external, it too can be easily removed and stored away safe.