What you’re looking for is a backup. RAID is not a backup, as another poster said it’s a tool for enduring high availability, and possibly higher throughput.
Buy a second pi and put it in another location in your house or even better at friends house then configure regular backups of your important data to it. There are also cloud services for doing backups which are great because having a location to do off-site backups to can be really hard to get as an individual.