Dear Blick, Weatheraardvark, and WeatherCat sys-admins,
Be extra cautious with upgrading to High Sierra because of the way Apple is forcing Apple File System with the upgrade. Take a look at this thread I started:
http://athena.trixology.com/index.php?topic=2566.0Apple is doing something that really needs to be bullet-proof and I'm personally very concerned about it because until now Apple has relied on something well-proven: the UNIX file system. Our Macs have hundreds of thousands of files typically and many applications use the file system as a sort of database, further overloaded the file catalogs. A UNIX file system very rarely gets corrupted, but OS X is now much more likely to have corruption creep into it.
The 64-question is how well Apple has designed the High Sierra file system migration to deal with the possibility that your existing file system has some corruption. Even if the High Sierra upgrades perfectly at the Apple store, if your file system gets seriously damaged in the attempt to upgrade, you may have a horrible time trying to recover.
Anyone contemplating upgrading to High Sierra should seriously consider first running at least Apple's Disk First Aid (preferably something more potent like Tech Tool Pro.) Once you've done everything you could to eliminate any errors in the file catalog, I recommend making sure your Time Machine backup is current, reformatting your working hard drive using High Sierra, and then restoring your computer from your Time Machine backup. Going about the long way will do everything possible to avoid losing anything in the change from the UNIX file system to APFS.
Cheers, Edouard