If you read the story you will hear about a test roll out an install. I was bitten by this and it was an awful couple of days of head scratching with Apple Sup

If you read the story you will hear about a test roll out an install. I was bitten by this and it was an awful couple of days of head scratching with Apple Support. My story is that I upgraded my mail server but the install hung never completed. I ended up with a a hard drive with no files and completely full. Diskutil and Disk Utility reported everything as good but there was a partition that could not be seen. I forget the eventual resolution. Probably starting all over again with a completely fresh install. I never understood what had happened until I saw a talk or read an interview with the APFS creaator. During the talk he spoke of how they did a complete re-format to APFS and verified everything and than reverted to HFS+ for the install. This is all from memory but I think I have the details mostly correct. The whole process was technically impressive but not without hiccups.