Single page Print

Western Digital's Caviar Black 2TB hard drive

Introducing the dual-stage actuator

Manufacturer Western Digital
Model Caviar Black 2TB
Price (Street)
Availability Now

The progression of the mechanical hard drive market is fueled not by ever-increasing spindle speeds, but by the availability of higher-density platters. Cramming more bytes onto each physical disk obviously enables greater storage capacities. At the same time, it also facilitates improved performance by allowing the drive head to access more data over shorter physical distances.

Conveniently, at least for comparative purposes, drive makers have settled on similar per-platter capacities over the last few generations. Sure, actual areal densities have varied a little, and each manufacturer has moved up the platter ladder at its own pace, but they've largely hit the same rungs. For example, Seagate and Western Digital both had drives with 125, 188, and 250GB platters. They went their separate ways through the 300s, with WD offering 320 and then 334GB while Seagate jumped all the way up to 375GB. However, it looks like the two are in agreement on the next battleground: 500GB.

Western Digital was the first to squeeze 500GB onto a single disk with the Caviar Green 2TB. Of course, the Green is one of those hippie low-power drives that's most comfortable sitting quietly in the corner, its reflexes hindered by a slower spindle speed and probably more than a few bong hits. That's just what you want for a closet file server, external enclosure, or home theater PC, but it's not really fast enough for a performance-oriented desktop.

Seagate's Barracuda 7200.12 cranked 500GB platters up to a full 7,200 RPM. However, the 'cuda was only available with up to two platters, capping the total capacity at just one terabyte. The 7200.12 wasn't the performance revelation many had hoped, either. Comparatively lethargic access times ultimately held back the drive's performance under more strenuous loads.

Now, it's Western Digital's turn to show what it can do with 500GB platters at 7,200 RPM. The company has brought four of those platters to the table in the latest Caviar Black 2TB.

The Black started shipping back in September, and it's been in and out of stock at online retailers since. As I write this, Tiger Direct does have drives in stock, although it's only one of a few e-tailers that do. Still, that's more than can be said for Seagate's 500GB/platter Barracuda XT 2TB, which was announced last month, but still isn't available for purchase.

I wouldn't be surprised if this latest Caviar Black was in high demand. After all, its predecessor, the Caviar Black 1TB, has reigned as the fastest all-around 7,200-RPM hard drive for quite some time now. The terabyte model's platters have only 334GB each, so the new Black represents a substantial 50% jump in per-platter capacity. And there's been an even more impressive boost in areal density. WD's 334GB platters shoehorn 235 gigabits into every square inch, while the new ones boast an areal density of 400 GB/in²—an increase of 70%. Obviously, the potential performance implications of such a leap are quite exciting.

Maximum external transfer rate 300MB/s
Maximum sustained data rate 138MB/s
Average rotational latency 4.2 ms
Spindle speed 7,200 RPM
Cache size 64MB
Platter size 500GB
Areal density 400 Gb/in²
Available capacities 2TB
Idle power 8.2W
Read/write power 10.7W
Idle acoustics 29 dBA
Seek acoustics 30-34 dBA
Warranty length Five years

Speaking of performance expectations, the 1TB Caviar Black is rated for a maximum sustained transfer rate of 106MB/s. The 2TB model's theoretical sustained peak is 138MB/s, which works out to a 30% advantage, but isn't anywhere close to the sort of linear increase one might expect. The fact is that data get a lot harder to access when you make them smaller, pack them tighter, and then have them rotate at close to an effective 120 km/h.

To help the new Black hit these smaller moving targets, WD equips the drive with a trick dual-stage actuator mechanism. The first stage uses a traditional actuator that, according to Western Digital, gets you into the right zip code. Stage two uses a piezoelectric motor to drive a second actuator that provides 500 nm of fine-tuning precision, taking the drive head down to the right house. 500 nanometers works out to about five tracks, and any so-called "short" seeks within that range can use the piezoelectric actuator without needing to hit the main one.

The piezoelectric-powered actuator is apparently too small to photograph easily (not to mention sealed within a casing I'm not inclined to tear apart just yet), but it's illustrated in the diagram below. Circled in yellow is the main actuator, with the secondary stage highlighted in blue.

Source: Western Digital

The piezoelectric actuator is more energy-efficient than the main arm, but because it adds a second motor to the drive, Western Digital says that overall power consumption is probably slightly higher than it would have been with a single-stage design. However, the company also claims this dual-stage approach is faster. For a drive like the Caviar Black, marginally higher power draw is a small price to pay for improved performance.

Western Digital tops off this latest Caviar with 64MB of cache memory, double what's available on other models in the line. The lower capacity points in the Caviar Black line haven't been upgraded to the denser platters just yet, either. That migration will happen over time, although it looks like model numbers won't change, making it difficult for end users tell which drives use which platters. At least with the 2TB drive, you know what you're getting.

Because the 2TB derivative stacks four platters, Western Digital outfits it with a StableTrac motor that secures the drive shaft at both ends rather than just one. The Black also inherits NoTouch ramp load tech, which moves the drive head right off the disk at idle rather than letting it rest on the outer, unused portion of the platter.

Like other members of the Caviar Black family, the two-terabyte model comes with five years of warranty coverage. Three-year warranties are typical for desktop hard drives, with five years generally reserved for high-end and enterprise-oriented offerings.