On multiple occasions I’ve seen people use the term “backup” when talking about RAID 1 or RAID 5 and that is not correct. RAID 1 and RAID 5 provide redundancy but they do not provide a backup. Redundancy protects you against downtime in case of a drive failure. It does not product you against user error (accidental deletion), data corruption, acts of god (a lightening strike frying all your gear), etc.,. That is what a backup is for. A backup should be an exact copy of your data that is ideally stored in a different geographical location and should be checked periodically for data integrity. For convenience, and extra safety, you could make one copy to keep onsite and another copy to keep offsite.
Harddrives seem like they would make great backups because they have a large storage capacity and are relatively inexpensive. The downside is that all harddrives will fail even if they are kept on a shelf (this might actually accelerate their demise). If you are going to use a HDD as a backup definitely make two copies.