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If anyone is searching for this. I ended up going with Duplicacy.  (First was Duplicati but the backups were slow and getting stuck.)  You just need to use the unassigned devices plugin. Then one of your internal drives can be an extra backup location and use Duplicacy/Duplicati or something else to sync there. 

But as alluded to above, this will help protect against your array failure, but not fires, theft, lighting strikes, surges, or the controller/PS frying your backup drive (though Duplicacy versioning should mitigate ransomware). But everyone should have multiple backups (one off-site) and not rely on one backup anyways.  I personally also backup to Crashplan (and plan on storing a drive at my brothers house) so it wasn't worth it to me to backup to USB for the very rare chance that controller card/PS fries 2 array drives and backup drive (and not USB drive), bother with expensive tapes or removable media, or build an entire other machine just for additional backup.  

Also: for email notifications duplicati hooks in well with healthchecks.io and you can use a emailToSMS gateway (check your email-to-number with your provider) to get an txt right to your phone for free whenever a backup doesn't complete