PC storage is split these days between solid-state and mechanical drives, although the two camps are complementary. SSDs offer blazing performance but are relatively pricey per gigabyte, while mechanical drives boast plentiful storage at much lower prices. The trick is to combine the two, which is fairly easy in desktop systems that can accommodate multiple drives.
Things are more complicated for notebooks, all-in-one systems, and small-form-factor rigs. Many of these machines are limited to a single 2.5" storage bay, eliminating the dual-drive option—or so we thought. WD's new Black² "dual drive" stacks a 120GB SSD on top of a 1TB notebook hard drive. The resulting combo measures just 9.5 mm thick, allowing it to slip into standard drive bays.
The Black² isn't the first product to marry flash and mechanical storage in a single 2.5" drive. Hybrids, these days known as SSHDs, have long augmented traditional hard drives integrated NAND caches. Those caches generally are relatively small, though, and they're managed entirely by software or firmware. Users have no control over what gets put in the flash.
On the Black², the SSD and HDD are separate entities. They share a single Serial ATA port, but Windows sees two distinct partitions. Users can put their OS, applications, and performance sensitive-data on the SSD and dump everything else on its mechanical sidekick.
WD really does stack the Black²'s SSD on top of a traditional HDD. Removing the top layer reveals a slim 7-mm drive that looks very similar to WD's Blue 1TB. This single-platter drive has a relatively sluggish 5,400-RPM spindle speed. It's a good thing there's an SSD riding shotgun.
The SSD pairs JMicron's JMF667H controller with 20-nm MLC NAND. The controller chip is a quad-channel design, so it has half the internal parallelism of the best consumer-grade SSDs. WD claims the SSD can read at up to 350MB/s and write as fast as 140MB/s.
The Black² has only been in my lab since Friday, and its unique configuration is incompatible with some of our usual benchmarks. I did run a couple of quick CrystalDiskMark tests to get a sense of the drive's performance, though. The SSD hits 420MB/s in the benchmark's sequential read speed test, and it manages 127MB/s in the write speed test. Compare that to 115/113MB/s for the mechanical portion.
There's a much bigger performance difference with random I/O. In the benchmark's random 4KB QD32 tests, the SSD scores 225MB/s for reads and 122MB/s for writes. The HDD is orders of magnitude slower: 1.6MB/s for reads and 1.0MB/s for writes. Friends don't let friends use 5,400-RPM hard drives without solid-state storage in tow.
Although the Black²'s SSD is listed as a 120GB drive, there's actually 128GB of NAND onboard. Overprovisioned spare area is responsible for the discrepancy, just like on stand-alone SSDs. The cryptic markings on the flash packages provide little clue as to their origins, but my money's on IM Flash Technologies. They're the only provider of 20-nm NAND on the support list for the JMicron controller.
The circuit board attached to the hard drive reveals a Marvell 88SM9642-NMD2 chip. Based on Marvell's product number conventions, it looks like the chip is a Serial ATA multiplexer, which would make sense given the fact that the Black²'s SSD and HDD elements share the same SATA port. The 6Gbps interface should provide sufficient bandwidth for both components. However, CrystalDiskMark reported lower scores when running on the SSD and HDD simultaneously. Performance evidently suffers when the Black²'s dual drives are accessed at the same time.
The sharing arrangement also comes with some other limitations. The Black² presents itself as a 120GB SSD; the 1TB mechanical portion is revealed only after WD's software is installed. That driver is Windows-only right now (everything from XP on up is supported), and it doesn't work with storage controllers from ASMedia or Nvidia. Also, users will need an Internet connection to download the software. WD provides a USB key that contains the required download link, but the software should really be included on a proper USB drive. At least the installer is simple to set up.
WD includes a SATA-to-USB 3.0 cable in the box with the Black², and folks will be able to download a special edition of Acronis' True Image software for use with it. However, WD recommends doing a clean OS install rather than cloning an existing system onto this drive. "You have to be a pretty advanced user" to successfully clone an OS drive, the firm says, and it adds that imaging manufacturer-installed recovery partitions wont work at all. I haven't attempted any cloning myself, but installing Windows 8 on the drive was a breeze.
Since my time with the Black² has been short, I'm hesitant to say too much about its performance. However, my initial impressions are positive. Windows feels very responsive running off the SSD. Applications install quickly, and load times are short. It's a lot like using a full-blown SSD. The Black² is nearly silent, too. And it has five-year warranty coverage.
There's just one catch: WD is charging $299 for the thing. For that much, one can buy a terabyte notebook drive and a larger 256GB SSD. The Black²'s 120GB flash capacity could be matched for about $100 less. Those configurations would require a second 2.5" bay or an expansion slot like mSATA or M.2, of course, but the resulting tag teams should be a fair bit faster than the Black².
For systems restricted to a single 2.5" bay, the Black² has some appeal. That said, traditional hybrids are a much cheaper single-drive alternative. Seagate's Laptop SSHD 1TB sells for only $120, for example. The Laptop SSHD has only 8GB of flash and is really a different class of product, but its caching implementation still does a good job of accelerating load times. Paying two and half times more for WD's dual-drive solution is a substantial step up.
WD says it created the Black² in response to SSHD critics who wanted more flash and more control over what goes in it. The Black² certainly addresses those concerns. I just wonder what those critics will think of the associated premium. I also wonder how many systems are actually restricted to a single 2.5" drive bay. mSATA slots have been around for a while, and the M.2 standard seems to be gaining traction. System makers should be including one of those slots alongside a traditional drive bay.
Just because they should doesn't mean they do or they will, though. Single-bay configurations exist, and they're especially common among older notebooks that are ripe for storage upgrades. The Black² allows those machines to enjoy the benefits of a dual-drive setup. Kudos to WD for developing an innovative alternative to SSHDs.