...high-capacity USB hard drive attracted a comment or three on the desirability, or otherwise, of larger-than-3.5-inch hard drives in the modern world.
The fastest spinning-disk drives today are 10,000 and 15,000 RPM 2.5-inch units. Western Digital's VelociRaptor line, essentially high-reliability server drives sold in the consumer market, remain a good compromise between performance, reliability and capacity. They're not as fast, especially in the latency department, as an SSD, but they're at least as reliable and have higher capacity, and much higher capacity per dollar.
("1Tb" VelociRaptor, formatted capacity 930-odd gibibytes: $US320 list price, perhaps a little less if you shop around. 256Gb SSD: around $US170, if you're not too picky about brand and specs. That's about three times the price per gigabyte as the spinning disk, and you're not even looking at the even-worse-value 500Gb-plus SSDs. You can easily drop $US700 for a good-quality 512Gb SSD today; that's 1.3 times the price-per-gig of a 256, and it doesn't even buy you one of the maximum-reliability single-level-cell units.)
The small platters in these high-RPM drives are all about reducing access time. Faster spinning speed gives lower rotational latency, because the heads don't have to wait as long for the part of the platter they want to spin up to them. Smaller diameter gives lower seek latency, because the heads don't have as far to go. Smaller platters also reduce air-friction heat production, which is a problem for super-fast spinning disks, and why nobody's even sold a consumer-market 15,000-RPM drive. (The VelociRaptors are 2.5-inch laptop-sized drives mounted in a 3.5-inch-drive-sized heat sink. They need it.)
I, too, would be happy to buy an outrageously high-capacity large-plattered drive with modern data density, even if its latency was awful.
The whole drive production chain, though, is currently geared for 3.5-inch and smaller platters, and the fantastically small tolerances in modern drives might have problems with larger platters. Platter and head-arm droop caused by gravity might be an issue.
Still, just imagine an eight-inch drive with the same data density as the drive inside the Seagate Expansion box. That is, I presume, a "3Tb" Seagate Barracuda, with three platters and six heads (PDF datasheet).
The spindle holes in the middle of larger platters tend to be bigger in proportion than the holes in smaller platters, but back-of-an-envelope calculations still suggest an easy formatted capacity of five terabytes for a 5.25-inch drive, and ten terabytes for an eight-incher, with the same platter and head count.
(Some drives have fewer than two heads per platter; that means only one side of a platter is being used for data storage. This may only be some BS market-segmentation thing, but just as CPUs sold at a low stock clock speed may actually have failed to operate correctly at a higher speed, one-sided drive platters may have failed testing of the un-used side.)
And that's even before you start raising the platter count.
Stack 'em up...
(Image source: Flickr user Cambridge Cat)
...and the sky's the limit!