those are called hybrid drives.....
Hybrid storage products monitor the data being read from the hard drive, and cache the most frequently accessed bits to the high-speed NAND flash memory. The data stored on the NAND will change over time, but once the most frequently accessed bits of data are stored on the flash memory, they will be served from the flash, resulting in SSD-like performance for your most-used files
Hybrid drives are faster then normal HDD but are slower then SSD.