My reply #5 explains how the _CCC SafetyNet folder and its 672 GB of files got onto my drive and the "trick" to avoid it. This will only happen
if you use a CCC backup drive as the
source AND use
Migration Assistant to move the files. You can find information about this at:
bombich.com/kb/ccc5/frequently-asked-questions-about-carbon-copy-cloner-safetynetBTW, when you select a volume, in the Volumes list, you should see a "Snapshot Creation and Retention" area in the lower, right-hand part of the CCC window. When you enable the CCC Snapshots, you have five editable fields that will specify how and when those files will be stored or deleted.
As for costs, I bought the mini in April of 2018. I also bought AppleCare for it. So, the only cost for the work was paid ~2.5 years ago. If this same problem occurs after next April, 2021, I will definitely be going a different way.
I hope to have a 1TB (OWC) SSD arriving by the end of the week. When that is installed, it will become the boot drive.
The mini is operating fine now, but I can't seem to set up FTP properly in the special weather reporting app.
Until I get that working, the weather site remains out of service except for off-site links.