As others said, depends on your use case. There are lots of good discussions here about mirroring vs single disks, different vendors, etc. Some backup systems may want you to have a large filesystem available that would not be otherwise attainable without a RAID 5/6.

Enterprise backups tend to fall along the recommendation called 3-2-1:

• 3 copies of the data, of which
• 2 are backups, and
• 1 is off-site (and preferably offline)

On my home system, I have 3-2-0 for most data and 4-3-0 for my most important virtual machines. My home system doesn’t have an off-site, but I do have two external hard drives connected to my NAS.

• All devices are backed up to the NAS for fast recovery access between 1w and 24h RPO
• The NAS backs up various parts of itself to the external hard drives every 24h
  • Data is split up by role and convenience factor - just putting stuff together like Tetris pieces, spreading out the NAS between the two drives
  • The most critical data for me to have first during a recovery is backed up to BOTH external disks
• Coincidentally, both drives happen to be from different vendors, but I didn’t initially plan it that way, the Seagate drive was a gift and the WD drive was on sale

Story time

I had one of my two backup drives fail a few months ago. Literally actually nothing of value was lost, just went down to the electronics shop and bought a bigger drive from the same vendor (preserving the one on each vendor approach). Reformatted the disk, recreated the backup job, then ran the first transfer. Pretty much not a big deal, all the data was still in 2 other places - the source itself, and the NAS primary array.

The most important thing to determine about a backup when you plan one - think about how much the data is valuable to you. That’s how much you might be willing to spend on keeping that data safe.