If you're using macOS Mojave

1. Power the device into recovery mode by pressing and holding Command & R keys. Release the keys when you see the Apple logo or spinning globe.

2. Once you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.

3. Type diskutil resetFusion in the Terminal window, then press Return (Enter).

4. Type Yes (with a capital Y) when prompted, then press Return (Enter).

5. When the 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. 

If you're using macOS High Sierra or earlier

1. Power the device into recovery mode by pressing and holding Command & R keys. Release the keys when you see the Apple logo or spinning globe.

2. Once you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.

3. Type diskutil list in the Terminal window, then press Return (Enter).

4. Terminal displays a table of data about your drives. In the IDENTIFIER column, find the identifier for each of the two internal, physical drives that make up your Fusion Drive. Usually the identifiers are disk0 and disk1. One of them should be 128GB or less in size. The other at least 1TB in size.
   

   1. If you need to unmount the disk then type diskutil unmountDisk disk# (disk0 or disk1, whatever the disk # is).

   2. If you need to erase the disk then type diskutil eraseDisk jhfs+ CAD\ HD disk# (If you need to view the filesystems type diskutil listFilesystem.)

5. Type the following command, replacing identifier1 and identifier2 with the identifiers you found in the previous step (ex. disk0 disk1). Then press Return.
   

   1. diskutil cs create Macintosh\ HD identifier1 identifier2 
      

   2. If you get a disk unmounting error, enter diskutil unmountDisk identifier, using the first identifier you gathered previously. Then enter same command again using the second identifier. 

6. Type diskutil cs list, then press Return.

7. Terminal displays additional data about your drives (volumes). Find the string of numbers that appears after ”Logical Volume Group” for the volume named Macintosh HD. It's a number like 8354AFC3-BF97-4589-A407-25453FD2815A.
   Example:
   +-- Logical Volume Group 8354AFC3-BF97-4589-A407-25453FD2815A
   |   =========================================================
   |   Name:         Macintosh HD
   

8. Type the following command, replacing logicalvolumegroup with the number you found in the previous step. Then press Return. 

   1. diskutil cs createVolume logicalvolumegroup jhfs+ Macintosh\ HD 100% 

   2. Example: diskutil cs createVolume 8354AFC3-BF97-4589-A407-25453FD2815A jhfs+ Macintosh\ HD 100%
      

9. When Terminal indicates that the operation was successful, quit Terminal to return to the macOS Utilities window. 

10. Choose Reinstall macOS, then follow the onscreen instructions to reinstall the Mac operating system. Your Mac restarts from your Fusion Drive when done.