To date all the hybrid storage solutions we have seen have been rather limited. Seagate got the idea right with the Momentus XT but having only 8GB of NAND (4GB in the first generation) limited the gains of caching dramatically. There simply wasn't enough NAND cache to fit all the data that users frequently used. Sure it was better than a traditional hard drive but the experience was far away from the real SSD experience. I'm actually surprised that there hasn't been any major evolution in the hybrid market -- the original Momentus XT was released over three years ago and fundamentally the current third generation Momentus XT is very similar. Back then I would've expected more and better offerings to be released within a year but obviously that hasn't happened. Until now.

WD Black² combines a 120GB SSD and a 1TB 5400rpm hard drive into one 2.5" 9.5mm solution. WD doesn't actually call the Black² a hybrid drive, but a dual-drive because the the SSD and hard drive are completely separate. In other words, the drive will appear as two separate volumes: 120GB SSD and 1TB hard drive. There's no caching involved and the end-user can decide what goes to the SSD as if it were a standalone drive. By default only the SSD portion is usable but WD supplies a USB drive with drivers in the retail package to make the hard drive portion visible to the OS as well. All Windows versions from XP and up are supported but there is no OS X support at the time of launch.

The performance specs above are for the SSD part. Unfortunately WD wasn't willing to disclose any hardware details about the SSD other than that it uses 20nm NAND but we will find out the details once our review sample arrives this week. I was, however, able to get the information that WD worked with a partner to bring the Black² to the market and that partner wasn't SanDisk (which was my first guess due to their prior partnership in hybrid drives). The actual hard drive spins at 5400rpm and consists of a single 1TB platter but WD didn't release any separate performance data for it. When the hard drive isn't in use, it can spin down to reduce power consumption, although 0.9W is still fairly high compared to most SSDs. 

Update: Techreport has already received their sample and opened it up, which revealed JMicron's JMF667H controller. We don't have any prior experience with the controller but generally JMicron's controllers have not been the greatest but it's been years since we've tested a JMicron based drive so things might have changed.

Update 2: WD initally told us that the drive is a single-platter design but they've now corrected their earlier statement. The hard drive consists of two 500GB platters as some of you suspected in the comments.

Given that most laptops only have room for one 2.5" drive by default, I do see a potential market for the Black². In the past consumers have had to make the choice between a fast but small SSD or a large but slow hard drive. The Black² eliminates the need to do that compromise. However, I'm disappointed for the lack of caching support. WD told us that they conducted market studies and according to those end-users wanted full control of the SSD and data. I don't completely agree because the reason why most consumers have negative thoughts about caching is because the available cache sizes are just way too small. Apple is the only one who has done it right with Fusion Drive by not going any smaller than 128GB, while others are trying to get by with 8-32GB. The Black² has enough NAND for a pleasant caching experience, so not including caching software is a letdown. Technically you could use third party caching software but I still would have hoped for a solution from WD, preferably something user-configurable so it's not forced like in the Momentus XT.