Here’s the summary for the wikipedia article you mentioned in your comment:

Fusion Drive is a type of hybrid drive technology created by Apple Inc. It combines a hard disk drive with a NAND flash storage (solid-state drive of 24 GB or more) and presents it as a single Core Storage managed logical volume with the space of both drives combined. The operating system automatically manages the contents of the drive so the most frequently accessed files are stored on the faster flash storage, while infrequently used items move to or stay on the hard drive. For example, if spreadsheet software is used often, the software will be moved to the flash storage for faster user access. In software, this logical volume speeds up performance of the computer by performing both caching for faster writes and auto tiering for faster reads.

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