Quoted from swampfire:
My friend, 1 is no redundancy. Learn from my fail, back up your NAS. I lost 2 years of cute kid pics. Don’t be like me.
Hah no, absolutely not. Your NAS can be a backup for all of your data from everywhere. But at least one other complete copy needs to be somewhere else, like kept on the drives of the original computers where they came from. Yes in theory the NAS raid setup will allow a recovery from a single drive failure and let you swap the drive and rebuild everything (if it is set up that way). But I wouldn't rely on it.
And, you still also need a separate backup of everything off site. Somehow, cloud or otherwise.
As a supposed computer tech person for years (which nowadays I do my best to hide from anyone and everyone but family and friends still know) I get people coming to me all of the time with their problems. So many times I have seen people lose data forever, things like photos, videos, etc. I have seen them lost due to hard drive failures, mobile device failures, etc. I always have to tell them, with no backup there is nothing I can do for you. If you have a hard drive failure there are companies that will take it in and perform a forensic recovery and they can probably get most of your stuff. But you have to pay thousands. This type of recovery was developed into an art for rustbucket spinning hard drives. Now with everything moving to SSD, there might be fewer failures. But SSD can still fail, all at once and with no warning, then you are in the same boat if you don't have a backup.
I have also seen more people than I can count who keep stores of things like photos on a single USB drive or SD card. Those things were never meant to be permanent storage reservoirs for important data, they will fail for sure. Then, everything is gone again. Although again often things can be recovered.
But I don't know how exactly so don't ask me...