It is fixed now, but it was an unusual fix …
And there is one thing that bothers me a bit still, but anyway …
I started the computer and looked at Process Explorer, and it was AvastSvc.exe that was hitting the hard drive for an average or 6-7 MB on read. AvastSvc.exe was not using much CPU or memory, so it didn’t seem like a likely culprit from Task Manager.
I updated Avast and the virus definitions and there was no change.
I then turned on silent mode and turned off all shields and there was still no change (which if I hadn’t checked with Process Explorer would have made me think it was not Avast-related).
Then I went into services.msc and stopped the Avast Antivirus service and the HDD went silent.
Then I restarted the service and everything was still fine.
Turned off silent mode and no issues since. (I also set the updates mode back to streaming and turned off the dial-up option.)
I then looked in services.msc and turned off automatic updates and set it to disabled - it was set to automatic.
Now, the weird part - I went into task manager and disabled WPFFontCache_v0400.exe - I wish I had checked the location in Process Explorer first. So far it hasn’t returned.
Then I went to delete the files, but …
There are NO WPF* files on the C:\ partition.
There are a handful of files on the G:\ partition that start with WPF, but NO WPFFont files. (I don’t mean that I deleted them, I mean that I didn’t find anything there TO delete.)
So I’m wondering what this “service” really was …
(I’m not a security expert, but it seems possible there was some malware pretending to be WPFFontCache_v0400.exe - but hiding from both Avast and MalwareBytes, which seems unlikely).