I think we're talking past each other. Both today, and in terms of potential, flash-based storage has a higher storage density, where storage density is the simple math of bytes/volume.

You keep coming back to "practicality", and this is harder to define. I welcome a proposed definition, so we can talk about this more clearly.

As a first pass, I might consider "practical to access" as being a combination of available at retail, and able to be readily attached to consumer hardware through a combination of onboard motherboard IO connectivity and expansion cards.

Here's a list of hard drives available at retail[0] that shows the largest retail HDD at 16TB and 3.5" form factor, vs SSD at 15.36TB and 2.5" form factor. These are available at retail today, and in standard form factors, with standard physical interfaces (SAS and SATA).

Without even getting into volume, but just looking at a single dimension, width, we can see that the HDD is 40% larger, physically and only 4% larger in storage capacity. The story only gets worse if we move to volume, because the 2.5" form factor is smaller in every dimension than 3.5". An enclosure to hold equivalent storage based on these two drives would be smaller for the SSDs than for the HDDs.

Perhaps practicality just means something that you could find without needing specialty hardware? Datacenter stuff is huge:

If we move to things you might find in a datacenter, we can find some articles (both from 2018). One talks about an existing 100TB 3.5" SSD - still in a standard form factor and with a standard interface (SATA/SAS).[1] In another article from the same year, Seagate was introducing the largest HDD, a 16TB model, and discussing the potential to hit 60TB by 2030.[2] Again, this is at the same time that a 100TB SSD became available.

Price is also something that has come up a couple times. I do not know if price is synonymous with practicality to you? I would appreciate clarification on the matter.

SSDs are, as I have mentioned previously, more expensive per unit of storage than HDDs. This is not up for debate, but I feel almost as if you think I am debating it.

So, for a specific use case, it is very easy for one to make a judgment call that HDDs are a better investment. But I do not see any argument to be made that HDDs are denser than flash-based storage. Again, density is storage capacity per unit volume.

A specific example, archival storage, would likely not benefit from an all-flash storage solution, but would be better served with HDDs. The tradeoffs for archival typically favor cost and often don't need the read and write performance of an SSD. As we agree, HDDs have better capacity per unit price.

If practicality and price are synonymous, it becomes a bit of a weird measure. We'll be taking a double ratio, of price/bytes/volume. But lets do it:

HDD dimensions(mm):
26.11*101.85*146.99
390891.021465

HDD density (TB/mm^3):
16/390891.021465
.00004093212461118814

SSD dimensions (mm):
15*70.1*100.45
105623.175

SSD Density (TB/mm^3):
15.36/105623.175
.00014542263097090198

HDD $/density ($/TB/mm^3):
499.99 / .00004093212461118814
12215100.11389283589280755309

SSD $/density ($/TB/mm^3):
4295 / .00014542263097090198
29534605.24902343799726872620

So, we've got an HDD at $12,215,100/TB/mm^3 and an SSD at $29,534,605/TB/mm^3. If this is the measure that practicality represents, then yes, HDDs have a lower number. If this is your definition, then I am sorry for misunderstanding you.

On the point of density, though, it seems clear to me, and I hope to anyone else looking at the data, that flash-based storage is denser (bytes/volume) than HDDs.

Dimensions are from Newegg[3][4].

[0] https://pcpartpicker.com/products/internal-hard-drive/#sort=...

[1] https://www.zdnet.com/article/worlds-largest-ssd-hits-100tb/

[2] https://www.alphr.com/storage/1010285/seagate-reveals-world-...

[3] https://www.newegg.com/seagate-nytro-3330-15-36tb/p/1Z4-002P...

[4] https://www.newegg.com/seagate-ironwolf-pro-st16000ne000-16t...