fusion hard drive not partitioning

How to fix a split Fusion Drive

Before you begin

If you're not sure that your Mac was configured with a Fusion Drive, or that the drive has been split:

1. Disconnect any external storage devices that are connected to your Mac.
2. Choose Apple () menu > About This Mac, then click the Storage tab.
   • If you see a drive labeled Fusion Drive, your Fusion Drive is working and this article doesn't apply to you.
   • If you have an Apple Fusion Drive that has been split, you should see two drives. One of them should be labeled Flash Storage, with a capacity of 24GB, 32GB, or 128GB. The other should be at least 1TB in size.

Fusion drives are notoriously difficult at being controlled/partitioned/whatever. Do you already have a partition created (for Windows, for instance)? If so, you can only have two partitions on a Fusion drive.

About Fusion Drive, a storage option for some Mac computers - Apple Support

If it does appear as having been split, follow the directions in the link provided previously.