For several years I have been comparing the price of a one terabyte (TB) hard disk drive with the price of two 500GB drives. After four years this is no longer a meaningful comparison, as the 1TB drive became more cost-effective two years ago. It makes little sense to continue my series. Maybe I'll do it again when the 1PB drive arrives, comparing it to two 500TBs.

One aside: The price of the 1TB drive has climbed since my last note. However, so has the 500GB drive. The cost-effectiveness of the 1TB remains. (The price rise is due to the floods in Thailand, where the majority of hard disk drives are manufactured. Supply has been compromised, so prices have risen.)

Recently a friend claimed that solid state storage (SSD) was the wave of the future and would replace rotating mass memory in five years. I agree with him as to the future but I thought his timing was very aggressive. I've decided to compare the 1TB drive with other forms of storage in order to expose the solid state trend and see if my friend is on target.

The last column, "HD Advantage," shows how much less a conventional hard drive costs compared to the least expensive other form of solid-state memory (excluding the hybrid drive). At the moment, rotating memory still holds a commanding lead by a factor of 10.

What about my friend's theory? The oft-quoted Moore's Law, interpreted loosely, says that the price of a given piece of technology 18 months from now should be half. If this rule is applied equally across all four memory types shown here, the ratios would be exactly the same in 18 months and hard drives would still have a 10x advantage.

The only way my friend can be right is if there is a different rate of advance in the various technologies. In other words, the cost per GB of hard drives would have to stagnate while the cost per GB of solid state drives would have to drop dramatically. So let's pretend that the cost of hard drives stays at 14 cents per GB while the cost of SS drives follows Moore's law. How many years would it take the SS drives to match rotating memory in cost?

The answer is 4.5 years. My friend's theory might have some legs.

But only if rotating memory stagnates and other considerations are ignored. The truth is that the hard drive manufacturers have consistently beaten Moore's Law. Based on a 500MB drive costing $500 in 1992, the Law predicts that the cost per GB in 2011 would be $4.23. As shown in the table, the price is actually 30 times better than that. If inflation is factored in, the advantage is even greater.

In all fairness, solid state memory offers some advantages. Electricity is one; with no motors and no moving parts, SSD uses much less power (less than 25% as much) and as a result requires less cooling. Over five years, this could save about $100 per drive. That's not enough to counter the much higher purchase price right now but it is green. Note that while hard drives can increase capacity while using approximately the same amount of power, more SSD capacity requires additional electricity. Lower power consumption is great for laptops on battery, a key advantage.

Other advantages include faster performance, almost instant start-up time, no latency and thus no need to worry about fragmentation, and much less likelihood of physical damage if the drive is subjected to external forces (i.e., being dropped).

One problem with SSD is that the memory technology employed has a limited number of write cycles. This is generally ignored for consumer-grade SSD devices. Enterprise-class SSDs have special algorithms that move data when a given area of the memory nears its end of life and are sometimes "over provisioned" (delivered with more capacity than specified) to compensate.

Perhaps I'll live to see the demise of the mechanical, rotating hard drive. Don't count on it.

Note: I generally check prices on 3.5" 7200 Seagate Barracuda hard drives, SanDisk SDHC Class 10 memory cards, Crucial's SSD products and Crucial's RAM products. Updated 02 Dec to include the Seagate Momentus XT 750GB hybrid drive.