How to revert your Fusion Drive back to HFS+ file system in macOS High SierramacOS High Sierra will not support Apple File System (APFS) on Fusion Drives at launch. If you converted to APFS in the beta, here's how to revert back to HFS+.
During the developer beta testing for macOS High Sierra, the disk format of some systems were converted to the up-and-coming Apple File System (APFS). The final public version of macOS High Sierra will not launch with APFS support on Fusion Drives. If your Fusion Drive is currently converted to AFPS, you'll want to revert back to HFS+ before installing the final build of macOS High Sierra. Here's how.
Note: This only applies to Macs with Fusion Drives**.
Before you start
What you need
Step 1: Download the latest macOS High Sierra install Assistant from the Mac App Store
Step 2: Create a bootable installer for macOS High Sierra
Step 3: Erase your hard drive and restore from the bootable installer
Step 4: Migrate your data from your recent backup
Important: Please follow Apple's support page for precise instructions on how t...
Source:
How to revert your Fusion Drive back to HFS+ file system in macOS High Sierra
