These days when I buy a new HDD (today it would be usually a 4TB from price per GB point of view) I automaticly also buy another 4TB USB drive for backup. xD Because the size of these HDDs are getting so big and there's potential for so much dataloss and I didn't fancy the idea of running expensive RAID setup nor a NAS. .
This is what i've done for a few years now. Seems pricey. But at the end of the day its cheaper to buy 2 drives then to recover one (recovery is expensive!). Though mine are just internal drives that i run backups of manually.