Apple File System in macOS High Sierra won’t work with Fusion Drives
MacOS High Sierra will come out of beta and roll out to the public next week. If you have previously installed the beta version, you may need to take extra steps before installing the release so your Fusion Drive-toting machine doesn't experience any negative consequences. Apple announced that the new Apple File system (APFS) won't immediately support Fusion Drives and will only support systems with all-flash built-in storage in the initial release of High Sierra.
Those who tested out the beta versions of macOS High Sierra had their Fusion Drives converted to the new APFS. However, support was removed from the most recent beta versions, and it isn't coming back with the public release of High Sierra. Apple provided a set of instructions to help those users convert their Fusion Drives back from APFS to the standard HFS+ format before installing the High Sierra update. The instructions include backing up data using a Time Machine, creating a bootable installer, reformatting the machine using Disk Utility, and reinstalling the operating system update.
APFS' lack of formal support for Fusion Drives has been a restriction of the new file system since it became publicly available in beta last year. Apple also released a support document about preparing your Mac for the High Sierra update, and it states that Fusion Drives and HDDs are not supported.