I recommend that you consider buying a lot of RAM, and using a RAM drive instead. It should be much faster than any SATA-based device (600MB/sec vs whatever your CPU to RAM bandwidth is).
They have been mostly displaced by enterprise SSD based drives that have high IOPs or big data solutions with transaction logs and distributed reads and writes.
Edit: the one thing I'd like from a ram disk for it to survive a reboot; poking about, most memory contents doesn't change across a reboot but AFAIK no OS has the ability to persist a RAMdisk across a restart.
That suggestion makes no sense. It's one thing to buy a SSD/PCI interface that allows you to plug in your RAM sticks. It's an entirely different thing to buy a brand new computer.
Your suggestion is like "do you need more disk space? Just buy a new computer."
..unless, of course - you are blessed with one of the devices that have RAM soldered on. I cannot express how much I dislike these devices.
No one presented such use case in this discussion.
Most have four and I'd say most people have two free.
2. For the cost of upgrading to larger RAM is prohibitive because you have to throw away your existing DIMMs and replace them with new-to-market DIMMs that are extortionate because they've only just appeared
You don't have to buy a whole new computer to add RAM to it, unless you're already hitting the limits of the motherboard, either slots or total capacity. And if you are, you should be able to get away with getting a new motherboard instead of a whole new computer.
Right.
Do you think that anyone interested in using RAM drives and willing to spend money on hardware wouldn't have already considered using freely available software RAM drive?
Maybe author is not super into "build a PC" movement, but know they need more RAM, so they ask the question that makes sense to them. I don't know, and I won't assume either, so asking basic questions first makes sense.
Maybe they weren't aware you could create arbitrary memory space on hard-drives via swap, so questions around that would be appropriate as well.
To answer your question though, devices like that have been made from time to time, but I don't think I've seen one since DDR2. It really would be easier to increase your system ram and run a software ram drive.
If you're already maxed out on ram, you would need to get a new system with more ram slots or higher capacity per slot, etc of course. If you get some off-lease server hardware, you can get tons of slots though.
If you're wanting to do something with old ram sitting around, you should probably find a marketplace to sell it. Or plan to hoard it until it's retro.