Start up in Internet Recovery mode by holding down the key combination command-option-R at the startup chime. Release the keys when you see a spinning globe.
Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.
When the OS X Utilities window appears, select
Utilities ▹ Terminal
from the menu bar at the top of the screen—not from any of the items in the OS X Utilities window.
Follow the procedure in this thread to split up the Fusion Drive. The long string of letters, numbers, and dashes that you will have to enter will be different in your case. Copy and paste it from the Terminal window into the command.
This operation will remove all data from the Fusion Drive, so you had better be sure of your backups.