Fast question, recommended usb3 external backup for NAS

I just purchased a synology 415+ and (3) WD Red Pro 2TB drives. I plan on putting them in Raid 5 which will net me close to 4TB of free space. It will be located in my basement. I'm looking to get an external usb3 hard drive that I will do weekly backups of the NAS and then store it in our safe not located in our basement.

My question is, what do you recommend as a good/reliable 5TB external that pairs well with the NAS that I just purchased?

I had a 2TB WD MyBookLive that lasted me 7...

Fast question, recommended usb3 external backup for NAS