Is there "danger" in removing Macintosh HD from my Time Machine as a back up? I want to use Seagate Backup Plus Drive
None of the items in the "Exclude these items from backups" will be backed up. Usually only the backup volume (i.e. the Seagate Backup Plus Drive) itself is in the list.

So including Macintosh HD in the above list is counterproductive if you want to backup Macintosh HD.