Apple created the Fusion Drive when large-capacity SSDs were unaffordable except to high-end professionals. A Fusion Drive pairs a large, low-speed hard drive (5,400 rpm) with a small-capacity, super-fast SSD, typically 24GB to 128GB. MacOS optimizes storage so that the most frequently accessed files are stored on the SSD.
You can run into trouble when the Fusion Drive’s SSD starts to falter, before it completely dies. While SSDs have no moving parts, they have an effective lifespan of a certain number of write operations per memory cell. Internal software manages “wear leveling” to make sure cells don’t stop allowing new writes prematurely by ensuring each cell gets an equivalent number of writes over time.
