Last year we reviewed OCZ's RevoDrive Hybrid, which saw the marriage of a 1TB hard drive and a pair of 50GB SandForce SSDs on a PCI Express card. Potentially achieving performance close to that of a SandForce SF-2281 SSD with a 1TB storage capacity was very appealing; however, there were some notable drawbacks that prevented us from recommending that solution.
Then on January, Crucial announced the Adrenaline SSD series. Not meant to replace their existing and well regarded m4 drives, the Adrenaline is a cache solution meant to work along your existing disk drive, using a 50GB SSD to
act as solid state flash-based cache for your larger primary hard drive.
Sources.