[Hikmer]

So in addition to the nas, i need another drive for backup?  Would that be a drive in a usb case that hooks up directly to the nas?  So if i go with 2x 2Tb drives in the nas, i need a 4Tb for the external backup?

You don't "need" a backup drive...you need an offsite backup solution.  It could be a USB external drive or it could be a cloud hosted storage solution (although some may argue you should do both and that a USB drive isn't an offsite backup).  The Synology will allow you to configure many different Cloud hosting storage services as a destination for a scheduled backup job.

[GentleBender]

My opinion is that RAID makes a failed HDD recovery much easier, but that also depends on how much data you have to restore upon HDD failure. Replacing a HDD in a 16TB RAID is much faster than copying a 16TB backup to JBOD (just a bunch of discs) setup. But as stated before RAID is not a backup.