For a long time, mainframes had hard disks with a set of heads on opposite sides of the platters. I'm not sure the use cases for spinning disks these days justify that kind of investment - they aren't (or shouldn't be) used in random-write-heavy workloads (as in frequently updated databases), but more for archival and, often, they sit behind a flash disk acting as a cache (I do that for my home server - a lot of write traffic never hits the disk because it's overwritten before being evicted from the flash)

Data verification to predict drive failure seems like a decent use case though. The drive is always spinning, so you essentially gain an extra data path to do read-verify-fix IOPs. Whether you can build this in cost-effectively is a big question though. But at rebuild times climbing to upwards of a week, being unable to do continuous health monitoring starts to get real problematic.

This would be cool, but it can be done with a single set of heads if the utilization is less than 100%.

Multiple sets of heads would be useful if the limiting factor is positioning.

BTW, I don't know how a multi platter drive records its disk blocks. Is a block contained in a single platter or does it spread across all platters and read/writes from all heads at the same time?

Dual arms would be handy if the drive had a RAID-like checksumming scheme between platters. If a platter is corrupted but is still readable/writable (it wasn't a head malfunction), the drive could rebuild itself without the help of a computer.

Even if it is a head malfunction, the data could be redistributed between the other platters, reducing the drive capacity.

The specifics of how data is physically arranged across multiple platters is undocumented, complicated, and varies between models. But with some clever benchmarks, much of that information can be inferred: https://blog.stuffedcow.net/2019/09/hard-disk-geometry-micro...

A hard drive will only use one head on one platter at a time. A single logical block will be contained within a single track on one platter. The next logical block will usually be on the same track or an adjacent track on the same platter. Seeking from one track to the next using the same head is generally a bit quicker than switching to a different head on a different platter and getting it lined up with a nearby track.

If you've got slow HDDs, the usual solution is to RAID them together. At that point your limiter starts becoming how fast you can slurp data down the line. RAID-0 would sort of emulate what you're talking about here.

That still trades off density for speed, having independent arms would be a huge benefit.

Would it still be cheaper than a similar SSD? Would it be cheaper than an SSD-augmented HD?

> That still trades off density for speed

It does not. RAID-0 has no duplication, it acts effectively like your independent arms. No density tradeoff, just speed.

There is a tradeoff: With RAID0 your parallelization for read or write is on the scale of 24TB chunks, not within 24TB chunks.