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My 2 TB Fusion drive has become a SATA Drive of 2 TB. I have an IMac 27 inch late 2015 with a 2 TB Fusion drive. After an update my computer crashed. I had to install the OS X from the OS that came with the IMac. I have updated everything to OS Catalina 10.15.7 and i use the Time machine back up to restore. But when i started the IMac i noticed that everything was soo slow.. `everything works slow. Opening apps, opening windows...slow. And then i noticed that my Fusion drive was a SATA drive! If u go to about this mac and then go to 'storage'. then u see that it is an SATA drive instead of Fusion. What can be happened? Is this why my. machine is so slow? Does the IMac not see the Fusion drive? ( i can't upload pictures here?..)

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A Fusion drive is simply software that presents two separate devices (a small SSD and a large HDD) as one disk volume.

However, it is possible for the drives to become 'split' as their separate components.

It sounds like your Fusion drive was split -- either when you erased and reinstalled after the crash; or possibly if the SSD failed. Do you not see the smaller SSD volume shown anywhere?

It's more usual for the mechanical hard drive to fail -- and I would be concerned about the reliability of a 7-year-old hard drive.

My advice is to get a large external USB SSD, and use that instead.

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Before restoring, you needed to fix your split Fusion Drive. Apple has instructions posted at How to fix a split Fusion Drive. Basic steps are given below.

These steps permanently delete all data stored on the drives that make up your Fusion Drive. Make sure that you have a backup before continuing.

If using macOS Mojave or later

  1. Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery. Release the keys when you see the Apple logo or spinning globe.
  2. When you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.
  3. Type diskutil resetFusion in the Terminal window, then press Return.
  4. Type Yes (with a capital Y) when prompted, then press Return.
  5. When Terminal indicates that the operation was successful, quit Terminal to return to the macOS Utilities window.
  6. Choose Reinstall macOS, then follow the onscreen instructions to reinstall the Mac operating system. Your Mac restarts from your Fusion Drive when done.

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