Take the external drive off site or use cloud storage. Stuff happens, e.g. fire or theft.

Don't use cloud storage that sync's the data two ways. Sync is NOT backup, it's replication. Backup and synchronous replication are not the same thing. Two-way replication is great for convenience and redundancy for your working copy(s) but it is not backup.

Use backup that copies one-way or the other. Files can be deleted, corrupted or even encrypted (search web for "crypto locker" virus) by a virus or hardware/software error. Multiple version iterations are better.