What does Hybrid Hard Drive (HHD) mean?
A hybrid hard drive (HDD) is a storage device that incorporates both the storage capacity of a hard disk drive and the performance and speed of a solid state drive by engaging rotating platters from hard disk drives and a small portion of high-speed flash memory together in a single drive. It monitors the data being accessed from the hard drive and uses 128 MB or more of the high-speed flash memory to cache the most frequently accessed bits.
Techopedia explains Hybrid Hard Drive (HHD)
When a computer boots, the operating system loads all of the data from the high-speed flash memory. This speeds up the boot-up time and saves power consumption because the drive doesn't have to be spun up to do so. Although the data stored in the flash memory will change, once the frequently accessed bits of data are stored, the data will also be loaded up from the flash memory, which results in better performance compared to an SSD.
Using hybrid hard drives or hybrid storage products is beneficial in terms of cost, capacity and manageability. Hybrid hard drives usually cost slightly more compared to hard disk drives, but less than SDDs because it is basically borne of both worlds. In terms of storage capacity, an HDD's volume could be as big as the traditional hard disk drive. Because the cache volume is hidden from the operating system, end-users do not need to decide on which data would be stored in the SSD because that is left to the drive controller and the OS.