Solution 1:

The most easy way to make a single disk from a broken disk, is to hold down option(alt) button on keyboard, so that you can access deploy studio server. Install fusion, sierra och admin. This will make a single disk if it is broken in to two disks. Then install mojave OS from app store as usual. Done!


Solutino 2: This solution is long and should be applied if first one is not working.



Follow these instructions  https://support.apple.com/en-gb/HT207584

OBS:

Instructions for Sierra and older versions worked perfectly.

Instructions for Mojave did not work.

So the best option is to convert the computer into older versions like Sierra, Mountain lion etc( but not Mojave). Then run terminal commands to make a single disk.




If you're using macOS High Sierra or earlier

           

  1. Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery. Release the keys 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 list in the Terminal window, then press Return.
  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.
  5. Type the following command, replacing identifier1 and identifier2 with the identifiers you found in the previous step. Then press Return.
    diskutil cs create Macintosh\ HD identifier1 identifier2

    Example: diskutil cs create Macintosh\ HD disk0 disk1

  6. 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. Then retry the command in step 5.
  7. Type diskutil cs list, then press Return.
  8. 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
  9. Type the following command, replacing logicalvolumegroup with the number you found in the previous step. Then press Return.
    diskutil cs createVolume logicalvolumegroup jhfs+ Macintosh\ HD 100%

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

  10. When Terminal indicates that the operation was successful, quit Terminal to return to the macOS Utilities window.
  11. Choose Reinstall macOS, then follow the onscreen instructions to reinstall the Mac operating system. Your Mac restarts from your Fusion Drive when done.