I have the latest 27" iMac. After a recent High Sierra beta formatted my Boot Fusion Drive to APFS, I attempted to revert the drive back to HFS+ in preparation for the release of High Sierra on the 25th. I used the routine outlined on the Apple Beta Software Program page. However, when I attempted to use step 8 (erase the boot disk using Disk Utility) the boot disk was rendered unavailable and I was subsequently not able to format it as either APFS or HFS+. Nothing I tried would work. Fortunately I had a backup on an external hard drive created by Carbon Copy Cloner. I was able to boot back into High Sierra using that disk. However, I am now having a little trouble restoring the iMac hard drive using Carbon Copy Cloner.