I have a very excellent SSD which I removed from that system because I am certain that the Windows installer would fuck it up, and I did not want the hassle of trying to fix it. So I pulled it out of the box to keep it from harm, and the only media I had at hand at that moment was a WD SATA HDD. I thought it would be slow, not kill-me-now slow.

I do not "use an HDD" of course. It was improvisational.

The same reason anyone has always used a HDD? … they're dirt cheap, compared to SSDs.

I'd consider hybrid being the best cost option, with a small SSD backing frequently used data, like the OS. But there's more complexity in that setup. I'm also a Linux user, and boot times don't bother me.

> they're dirt cheap, compared to SSDs.

If you need very large (4TB+ drives) maybe, but 1-2TB SSDs are so cheap nowadays. 2TB SSDs today are cheaper than 2TB HHDs from 10 years ago, and the price discrepancy is quite narrow unless you're looking at 4TB+ drives.

I don't even bother looking at HHDs for my own computers anymore unless I need bulk storage for videos or something.

Dirt cheap in which measure?

I was at Microcenter and some 1TB (rather questionable) NVMe drivers were $30 on special. Going to be difficult to get cheaper than that.

Now, lets turn your equation around. What is the cost per IOPS of your HDD versus SSD? HDDs start to get expensive very fast in that measure.

In terms of $/B.

Yes, HDDs are slower than SSDs. If that axis matters to you, you'd use an SSD, particularly NVMe. (Which is sort of implied by the hybrid setup I suggest.) If storage capacity matters, HDDs. You can see this reflected in market prices, though it does look like SSDs are surprisingly cheap these days, comparatively. Historically this has not been the case. (I wonder if economies of scale are now working against HDDs suddenly, or what? There's no reason for them to cost the same or more than an SSD — the market would collapse. Although I swear market pricing for many components hasn't made a lot of sense, recently… i.e., RAM has seemed horrendously expensive.)

There's some sort of big SSD price drop in the past 3 months. I dunno what that's about, but I did upgrade a machine, so that's nice.

There does definitely seem to be a pricing mechanic in that hard drives never really scaled down in minimum unit cost; the basic parts of a hard drive still cost real money, so if you can do 2TB per platter, and a top of the line drive has 10 platters, a single platter 2TB drive costs a lot more than 10% of the top of the line drive. OTOH, flash controllers aren't that expensive and/or the cost of the controller scales with the capacity, so SSD prices tend to be more linear with capacity.

If you need a lot of space, $/B means a lot, but if you just need an ample amount of space, $/device is more important, and SSD drives have hit the point where an ample amount of space is available for less than any hard drive.

The most efficient cost option is to have one cheap SSD for booting and a handful of apps that need the speed and then using a HDD for storage. Been that way for 10+ years