A Japanese company named DTS has added 1 GB DDR RAM on-board cache to hard disks to boost data transfer rates as high as 110 MB/sec. The hard drives, named as Mcell, will initially be available in 80 GB, 120 GB, and 160 GB configurations.
A quote from the article at Ars Technica:
The Mcell is an interesting beast. In terms of size, it fits in the standard 3.5" form factor, but inside it actually contains a 2.5" 5400 rpm hard drive, a stick of 1 GB DDR2 RAM, and a special chip from DTS that contains a real-time OS and CPU. Don't let the slow RPM fool you. The combination of the write-through cache and the 2.5" disc should afford much quicker access times than with 3.5" 7200 rpm drives.
Caches are used on hard drives to boost performance, and mostly NAND flash memories have been used for this purpose. With the larger cache, drives can read/write with larger chunks of data, hence improving performance. The high transfer rates are radically more than those achieved via usual SATA hard drives.
More news links:
RAM cache makes hybrid hard drive fly (Tech World)
DTS unveils hard drive with DDR RAM cache (Techspot)