One thing I am starting to see in this discussion is you can't really trust any technology for the long run.
In my experience, technology is more reliable than humans, especially if you put a little bit of design care upfront.
My biggest recommendation is: make sure all the backups are completely hands-off automated. My family has lost some theoretically backed-up data twice in 35 years. Once was my error during transferring from my college account to my own PC (ftp'd a zip file in text mode and detected it far too late) and the other was my Dad's hard-drive crash where the backup strategy was "every week or so, put in a DVD-R and double-click on this batch file on your desktop." When the drive failure happened years later, the most recent backup was over 30 weeks old.
Perhaps what I really want is several different ways of backing up in case one or two of them become obsolete.
Any of the commercial off-the-shelf NAS vendors I wouldn't worry about this. All of the ones I've seen (and definitely Synology) are using standard Linux (or BSD, which is similar) storage capabilities. You aren't going to unexpectedly have a storage means go obsolete and fail in a window where you can't easily get access at your data.
But don't know if I can afford too much of that.
That's often the problem in engineering. If you have a concrete budget and storage volume size in mind, sharing it might help people guide you to solutions that might fit. If you could spend $X now, $Y/month, and periodically add $Z in purchases, we could also suggest a "buy this now to start, in m months, add this to get a better level of data durability/availability".
I was looking at some old files tonight and saw that I have stuff that goes back at least 20 years and I haven't lost it yet. I have some floppies around with even older stuff, most of which is probably impossible to access. Perhaps I am over-thinking this. The data seems to last longer than the means to access it.
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Do you have any definite system in mind? One that would give me the advantages of a NAS as well as a good back-up scheme. And which will not be unsupported five or ten years from now.
If you can swing it, I'd get a 8 bay Synology NAS (DS1821+) or 5 bay (DS1522+) and put 4 drives in it in SHR-2 (which can survive two drive failures). If that's a little too rich, get a used Synology on Ebay (DS916+ is a little over $300 used) and put 4 drives in it. With SHR-2, you lose the useful capacity of 2 of the drives in service of redundancy. You can migrate to new drives [one at a time] over time, which I just completed this spring in my unit and it was easy.
Create two shares on that: one for critical (3-2-1) data and one for less critical (3-2) data.
For the critical data, plan to have it on your computer, on your NAS, and backed up offsite. Backed up offsite could be Google drive, Dropbox, a friend's NAS, whatever. That's the data that you'd really be frustrated to lose.
For less critical data, you might plan to just have it in two places, one of them being the NAS.
All of that is predicated on someone who wants to pay a little more and do a little less tinkering/learning. There is a cheaper path available that looks something like "get an old $100 desktop, put 4 drives in it, install unRAID, Proxmox or TrueNAS Core or TrueNAS Scale, and build the same type of functionality from more raw components". I'm in IT, started down that path, and then plunked down for the nicely packaged finished product and rely on that for the core of my storage needs here. unRAID and TrueNAS offer similar levels of "nicely polished" on the software side while you cobble together the parts on the hardware side. You can save quite a bit of cash upfront this way, but will likely have higher power bills month after month.
tl;dr:
flush with cash? Buy two Synology units, one large and one your-choice. Put the large one at your place and other one someplace offsite. Configure some (all?) data to replicate off-site. Consider backing up some to Google drive, backblaze, Dropbox or other cloud.
less flush? Buy one Synology and use a cloud vendor for the off-site third copy.
less? Buy a used Synology and do as previous
less and you're willing to have a minor side-hobby? Look into TrueNAS or unRAID.
less and you actively want a side-hobby? Look into proxmox
Sounds like you have had some bad experiences with drives failing at just the wrong time. Better grade of hard drives is required. That is also a common suggestion in this NAS business. I think I am being convinced of that.
I wouldn't buy bottom of the barrel drives, but I'm not entirely convinced that "datacenter grade" is needed in spinning hard drives for home users on a budget. I'd much rather design around "things break all the time, make sure the data is resilient" than to pin my hopes on better hardware failing less often. (That's a good
additional layer if you can afford it, but isn't the primary.) I'd trust a two-drive redundant array of commercial quality disks than a one-drive redundant array of datacenter-grade disks. The real takeaway for me is "one drive redundancy has the possibility to screw you if a fault goes undetected on drive X and then drive Y fails". That's why two drive redundancy isn't overkill, even though it seems like it at first. Data scrubbing is also important and something that I have set to run automatically every month (built-in feature of most systems).