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As your Mac is still supported by Apple, you should be able to obtain a replacement Fusion Drive, although that will need to be fitted by an Authorised Service Provider, and will possibly cost close to the resale value of the Mac. A better option, available at lower cost, would be to replace it with an SSD, which will also be faster. The most common problem doing that is the Mac then struggles to update its firmware with its new internal storage. You may be able to sort this by installing new versions of macOS on an external disk, which also updates the firmware in the Mac itself, but some Macs refuse.