The Kers systemstores energy that would otherwise be wasted as heat during braking, then reapplies it during acceleration to provide a boost in power, controlled from the cockpit by the driver.
These approaches can combine a smaller SSD that stores the operating system and applications and a larger HDD that is used for user data, or a hybrid HDD that has flash memory inside a HDD that is used to store frequently accessed data on the flash with less used data kept available on the HDD.