I followed the instructions on the post two days ago and ran "sfc /scannow" as administrator.
It took about 20 minutes.
Then I browsed C:\windows\logs\CBS\CBS.log. It looked like it repaired all errors but one with prncacla.inf, at the very end. The message says some corrupted files might not be repaired. This may relate to printer drivers.
Windows 8.1 on the C drive with Properties and Tools tells me that it find no errors on the drive now.
I will have to look into this.
One reference from SYSNATIVE is here. Another, from Microsoft, on hash mishmash, is here.
This is a developing issue. I will post more as I learn more.
Update: Aug. 26.
There are sporadic symptoms: a hang, or a forced restart with a hard drive error
HP's support instructions now seem to be this and this.