Brian's Brain

I've frequently written in the past, both online and in print, about the differing cost-per-bit and floor cost dynamics of hard drives and flash memory. In summary, a hard drive is burdened by the fixed cost of its mechanics; platter(s), head(s), motor, chassis, etc. But, on one or both sides of that platter (or those platters), it's possible to store a tremendous amount of data. Conversely, flash memory-based mass storage is roughly linear in cost with the number of required array transistors per component (a trend which multi-level-cell storage obviously influences, however), and the number of required components, needed to implement a particular amount of capacity.

So hard drives are significantly cheaper than flash memory on a cost-per-bit basis, but that metric is only relevant above a certain capacity threshold. And, as flash memory cost-per-bit decreases thanks to Moore's Law, and as hard drive cost-per-bit decreases thanks to perpendicular magnetic recording and other evolutions-and-revolutions, the threshold that delineates the capacity regions where a flash memory array or a HDD is cheaper in absolute cost moves upward over time. 'Wild cards' that may also influence the HDD-vs-flash decision include the fact that semiconductor-based storage tends to be more rugged and power-thrifty (especially in random read mode) than mechanical storage. And, of course, the cost-per-bit of a HDD also depends on what form factor HDD we're talking about.

For grins, I thought I'd collect a few days' worth of pricing data from DealNews and Techbargains to drive home these points. Note that many of these prices are AR (after rebate), but I think they're still relevant:

1) With the exception of occasional 'fire sales', the most cost-effective 3.5" HDD capacities are 250 GByte and 300 GByte variants. Roughly $80 for the former and $100 for the latter; ~33 cents per GByte in both cases, and sometimes the vendor even throws in an external enclosure at little-to-no price increment.

2) What about 2.5" HDDs? I've got two data points for you; a 60 GByte model for $89.99, and an 80 GByte drive for $99.75. Note two trends; 2.5" HDDs are more expensive than 3.5" HDDs on a cost-per-bit basis, and within the 2.5" family, 80 GByte drives are cheaper per-bit than 60 GByte drives. On that latter point, though, I suspect that if I were to further price-log 100 GByte and bigger drives, the cost-per-bit trend would sooner or later reverse course and head back skyward.

3) And 1" HDDs, in CompactFlash form factors? I've got two 4 GByte data points for you; $74.95 and $89.99. An 8 GByte 1" HDD, conversely, goes for $149.99. Reiterating the points made above, 1" drives are more expensive than 2.5" drives on a cost-per-bit basis, and an 8 GByte HDD is either cost-per-bit equivalent or superior to its 4 GByte siblings, depending on which sibling you choose.

4) Now let's look at flash memory-based CompactFlash cards. 1 GByte variants are $47.95. A 2 GByte card is $75. 4 GByte variants are $164.99. And 8 GByte flash-based CompactFlash cards? A whopping $329. Realize that these cards come from Sandisk's Ultra II and Lexar's Professional lines, which deliver high write performance (and are likely, therefore, based on single bit-per-cell flash memory; other, cheaper cards are based on more economical, albeit lower performance, MLC flash memory).

Particularly focus on bullet items 3 and 4 above. If all you need is 1 GByte of capacity, you'll go with flash memory, both because the smallest available HDD is more expensive and because it provides 4x the amount of storage you require. At 2 GBytes, the total cost of flash memory-based storage achieves rough parity with the floor cost of the 1" HDD. And at 4 GBytes and above, it's no contest; HDDs are the total cost leader. That is, unless you're comparing against 4 GBytes of MLC flash memory, which I suspect would hold its own against a 4 GByte HDD (but the HDD would still take the lead at 8 GBytes).

You know what's really wild about analyses like these? A year, a quarter, a month or even a week from now, the conclusion might be completely different, based on the at-the-moment supply-versus-demand status of flash memory and HDD, which after all are both commodities. Isn't predicting the future fun?