Backup strategy for readynas 516 not involving another NAS

I've owned three readynas devices over the past 10 years  and have avoided any major disasters  so far.  They say that when you back up your data that you should  back up regularly store off site.  So my question is this!  Is there a cost-effective, low-cost yet reliable solution for backing up the 4 -5 TB of data on my device that doesn't involve backing another raid device.  I did buy a FireWire drive a few years back but it topped out at 2 TB.  I'm wondering if someone could recommend  a device that is reliable but doesn't come with a cost premium for data availability  and redundancy that a raid device encompasses.  I notice on the back of the NAS there is a port for a LUN.  Could  this point me towards my backup strategy ?