The first generation of hybrid drives from Samsung and Seagate were released in 2007 in conjunction with Windows Vista. While these first-generation drives included a little bit of flash, they didn't have the CPU horsepower to effectively use it as a cache. Instead, they relied on Windows to manage the flash through its ReadyDrive feature. These drives never achieved any market success, and the world forgot about the whole concept for a couple of years.
Seagate's Momentus XT--now in a second generation that combines 8 GB of SLC flash and a 500-Gbyte or 750-Gbyte 7200RPM hard drive, along with greatly improved caching software, has been more successful. Some adventurous folks, myself included, have swapped out the hard drives in our notebooks for these hybrids because we were unable or unwilling to buy an SSD big enough for our needs. At present, I don't see the notebook vendors advertising the advantages of hybrid drives, but I hope that will change as SSHDs become more widely available.
The new Western Digital drives come in two form factors: a 500-Gbyte version at 5mm z-height for Ultrabooks and similar thin systems; and a 750-Gbyte, 7mm z-height version. Both have 32 Gbytes of flash and spin at 5,400 RPM. Meanwhile, Toshiba uses a 5,400-RPM disk with 8 Gbytes of flash in a 9.5mm package. Both vendors assume that the drives' lower random I/O performance will be hidden by the flash cache, and that users and OEMs will value the lower power consumption of the slower rotation speed. All things being equal (and, of course, they never are) I think Western Digital's design with more flash and a slower disk is probably the best combination of the bunch.
Some of my fellow storage analysts expect array vendors to adopt SSHDs, but I can't see it happening. While disk array vendors could speed up disk design by simply replacing standard drives with hybrids, they'd still have a limited range of drives to choose from, even with the addition of Western Digital and Toshiba to the market. Using a combination of SSDs and spinning disks gives designers much more flexibility and allows them to use significantly more flash than the 1% to 5% typical for a hybrid drive.
I think the market for SSHDs is limited to the notebook market, where the small size and reduced power consumption of an SSHD outweigh flexibility. I expect PC desktop vendors to follow Apple's lead with Fusion Drive that uses software to combine the performance advantages of an SSD and the capacity of a hard drive. PC OEMs could use Intel's SRT, Samsung's Dataplex or Condusiv's ExpressCache to use an SSD as a cache option. Should it become popular, I'm sure Microsoft will build SSD caching into Windows 9 or 10.
That said, the biggest threat to hybrid drives in the notebook space is the rapidly growing capacity, and rapidly falling cost, of flash-based SSDs. As flash controller vendors like IDT, Densebits and Anobit (acquired early last year by Apple) enhance the digital signal processing and error correction in their controllers, the price of SSDs using the new controllers and lower-cost TLC flash will come down significantly. Once SSDs cost just two to three times what a basic hard drive costs for similar capacity, performance-oriented buyers may simply skip SSHDs altogether. SSDs have already closed the capacity gap, with 960-Gbyte SSDs hitting the market from Micron at a cost of just $600 compared to $150 or so for a hybrid. As SSDs continue to boost capacity and cut costs, they might squeeze SSHDs right out of the market.