A hybrid drive, hybrid hard drive (HHD), or hybrid hard disk drive (H-HDD) is a type of large-buffer computer hard disk drive. It is different from standard hard drives in that it integrates a cache using non-volatile memory or even a small solid-state drive (SSD or SSH drive). Although the cache typically uses non-volatile flash memory, some drives use battery-backed volatile RAM (a hybrid RAM disk). The flash memory buffer can speed up repeated reads to the same location; a RAM buffer speeds both reads and writes, but must be written to backup storage before power is lost.
Unlike most standard hard drives, the hybrid drive in its normal state has its platters at rest, with the motor not consuming power or generating heat. Any data written to the hybrid drive is written to the buffer; there is no risk of data loss in the event of power failure as the buffer's flash memory is not volatile. When reading data from the platters extra data is read and stored in buffer memory (which need not be non-volatile flash memory) in the hope of anticipating future requirements as in any disk cache. Data required for the next boot-up can be stored in the non-volatile buffer before shutting down the computer.
The hybrid drive's platters will spin up to move data from the write buffer to the platters when nearly full, and to read data which is not in the cache.
Early estimates place the actual hard drive usage (when the platters are spinning) at anywhere between 1.25% and 10% for typical single users, although there are obviously situations where platter usage will be much higher, such as the encoding or editing of very large video files, producing reports from a database, etc.