Even this discussion of storage media is entirely orthogonal to RAID. Flash fails too! There is value in redundancy in flash storage applications.

Good point. I think it might be more important with SSDs actually when they fail they typically fail 100%. When HDDs fail you can sometimes get some data off or even use software tools to repair the disk(s) and get all your data back.

There are a variety of real-world SSD failure modes that depend on the specific controller, in addition to environmental factors (NAND doesn't like heat). I would not suggest using this reductive failure model for storage planning.

It’s actually the other way round I have found. I suggest you have a look around at other sources. Rust tends to go clunk at spin up time and that’s it. SSDs soft fail first due to reallocations but are still readable.

Both have very different failure modes depending on what is failing. Controller / electronics failure tends to instantly brick either. Some hard drives experience a single failure event that wipes out a bunch of sectors, but they work fine afterwards and if you overwrite the affected sectors those are back to being fine as well, because they are relocated. For other drives this is ongoing, were more and more of the drive becomes unreadable and even unwritable. I believe both of these are platter surface damage, the latter perhaps R/W head or amplifier degradation. Bearing and motor issues tend to make themselves known in advance, if you listen, but at some point it will stop spinning up or seize while running; instant loss (but a high chance of data recovery, since both media and controller are intact).

For solid state drives a lot of the issues are firmware problems. Things like drives becoming very slow over time have been caused by things like imbalanced internal data structures. Some are media related, e.g. some drives experience slow reads on old writes, because the voltage levels have decayed which necessitates internal retries or slower read speeds. Things like SSDs failing to enumerate are probably controller issues.

In my experience both can fail either slowly, with degraded performance, or fast and total. However, known-good SSDs (most of Samsung, Intel) tend to be very reliable. Good hard drives too.

Purely anecdotal, but I've never had a hard disk just outright fail 100%, they always start going click click click on certain areas of the disk, and avoiding those files you can rescue the rest. SSDs on the other hand, I've never had one properly go into a "read-only" soft fail mode, they just fail to show up one day.

This is all among consumer drives though, enterprise stuff probably behaves better?

I'd love to see some kind of statistics on actual failure modes, but aside from Backblaze, everyone keeps their storage statistics private.

Add a NAS unit to that cost and the recovery time, dire random access time to your estimations and it works out.

I’ve had to bounce around terabytes of crap on spinning rust for years. One nice XFS to XFS copy job took 2 days due to small files. Same thing on SSD was 3 hours.

I’ve also dealt with restoring a hosed 4 TiB NAS from another hard disk array. 4 days recovery sound like fun?

You're still conflating RAID and NAS. These things are orthogonal.