In the first beta seed of macOS High Sierra, Apple made it so that its new file system (APFS), supported Fusion Drives in iMacs.
In later beta seeds, however, Fusion Drives were dropped from the support list. While it was suggested that Fusion Drives may gain support again in the future, at least by the time the macOS High Sierra upgrade arrived to the public, that is indeed not the case. Apple has confirmed that the APFS will not support Fusion Drives in iMacs.
The macOS High Sierra golden master was released earlier this month, and the public launch of the desktop software is set to arrive to the public on Monday, September 25. In advance to that, Apple has published guidance to help iMac owners convert from AFPS back to the standard HFS+ format. The list of instructions is not short by any means, which includes creating a Time Machine backup and creating a bootable installer. By the end of the process, users will have to reformat their iMac and reinstall macOS High Sierra.
Interestingly enough, Apple’s own guidance says that the Apple File system will not be supported “in the initial release of macOS High Sierra,” which makes it sound like the APFS could be supported by Fusion Drives at some point in the future. However, when, or even if, that happens is unknown.
You can find the guidance for reverting back to HFS+ through the Apple source link below.
macOS High Sierra officially launched to the public on September 25.[via MacRumors; Apple]