The flash memory in the Fusion Drive moves data and applications frequently accessed by the user to the flash memory and move less frequently used data to the hard disk drive as the flash memory becomes full.

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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.

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Using a combination of HDD and flash memory in either a hybrid HDD or a dual storage notebook computer, where frequently used data (and possibly the operating system and application software) is kept in the flash memory, while the bulk of user content is kept on the HDD, has been shown to provide about 80% of the performance improvements of a pure SSD-based computer.

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Apple introduced its version, called Fusion Drive, that tiers storage on flash memory or a HDD depending upon frequency of use, but appearing to the user as a single storage device.

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