Fusion Drives are made up of two physical drives your system has within it at the OS level. You have a Blade SSD which is on the back side of the logic board and the 3.5” SATA HDD to the side of the logic board. The Blade SSD is used as a cache drive to the HDD as such it is hidden to the user.
Apple altered things a few times so depending on your OS level you will encounter different ways
Follow this to break the Fusion Drive with Sierra or High Sierra: Split Your Fusion Drive Apart
Here’s a good reference on the newer macOS Fusion drives: Fusion Drives in APFS which you don’t need to break apart.
But what about the data on the caching SSD??
As the SSD is just a cache drive it only holds copies of the data from your HDD as long as the system was shut down cleanly the data the SSD holds is duplicate of what the HDD holds.
So what’s important is whats on the HDD! So handle it with TLC.
So how do I recover the HDD’s data??
You’ll want to use a 3.5” HDD case or Universal Drive Adapter to hold the drive so you can then use Apple’s Migration Assistant to transfer your stuff over to the new drive! How to transfer data to your new Mac from your old Mac
To be clear you don’t want to re-establish the Fusion Drive! The small impish SSD is too small to be useful as a regular drive and is not faster than the SATA SSD you are putting in. All you’ll do is slow your system down!
For reference review the blade SSD’s performance here: The Ultimate Guide to Apple’s Proprietary SSDs I would just format it as a GUID drive and leave it be (unused) or replace it with a bigger SSD.
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