You'd want to couple this with some ability to check data integrity and you'd still need an offsite backup of some kind—plus you'll need a third copy anyway to

You'd want to couple this with some ability to check data integrity and you'd still need an offsite backup of some kind—plus you'll need a third copy anyway to recover if your primary fails and you plug the cold backup drive in only to find it's dead, which isn't unlikely enough not to worry about, or one or more files are corrupt, or whatever. Some inexpensive cloud storage service or a second (ideally different brand) cheap external stored elsewhere would do, though the latter's really inconvenient.