If you want to make proper backups, you need at least three copies: one off-site, one on-site backup, and one on-site backup that can be in the process of being written. That way, if something happens when making a backup, you will still have a previous backup from which to restore. And you will have the off-site backup as well (which can periodically be rotated with your on-site backups).
Note that this doesn't really provide much in the way of versioning, so you might want to add more copies as well (this is especially important as a defense against ransomware, for example, or even just a file that is accidentally deleted or changed, but not noticed until several backup cycles have passed).
In any case, it is important to consider what is being backed up, how big it is, how often it changes, and how important it is. Consider, too, that data recovery services generally start in the low four-figure range and are often ineffective with modern solid-state disks (i.e. backups are cheaper than data recovery).
External hard disks are a good start. Consider cloud backup services as well (but not as a replacement for physical copies that you own). For large volumes of data, magnetic tape is an option, too, although it has a high initial cost for the hardware.
OP: give us more detail about what you are trying to do and we can give you better recommendations.