To be honest I never understood the purpose of RAID for personal use cases. RAID is not a backup, so you need frequent, incremental backups anyway. It only makes sense for things where you need that 99.99% uptime. OK, maybe if you're hosting a service that many people depend on then I could see it (although I suspect downtime would still be dominated by other causes) but then I go over to r/DataHoarder and I see people using RAID for their media vaults which just blows my mind.
RAID is not backup, but in some circumstances it's better than a backup. If you don't have RAID and your disk dies you need to replace it ASAP and you've lost all changes since your last backup. If you have RAID you just replace the disk and suffer 0 data loss.
That being said, the reason why I'm afraid of not using RAID is data integrity. What happens when the single HDD/SSD in your system is near its end of life? Can it be trusted to fail cleanly or might it return corrupted data (which then propagates to your backup)? I don't know and I'd be happy to be convinced that it's never an issue nowadays. But I do know that with a btrfs or zfs RAID and the checksuming done by these file systems you don't have to trust the specific consumer-grade disk in some random laptop, but instead can rely on data integrity being ensured by the FS.
You should not propagate changes to your backup in a way that overwrites previous versions. Otherwise a ransomware attack will also destroy your backup. Your server should be allowed to only append the data for new versions without deleting old versions.
Also, if you're paranoid avout drive behavior, run ZFS. It will detect such problems and surface it at the OS level (ref "Zebras All The Way Down" by Bryan Cantrill)
i use mirror raid on my desktop. the risk of a disk dying is just to high. i even made sure to buy disks from two different vendors to reduce the chance of them dying at the same time. for the laptop i run syncthing to keep the data in sync with the desktop and a remote server. if the laptop dies i'll only be a few minutes out.
when travelling i sync to a USB drive frequently.
for the same reason i don't buy laptops with soldered SSD. if the laptop dies, chances are the SSD is still ok, and i can recover it easily.
