I don't remember exactly when it happened, but one day I fired up Parallels and my entire system ground to a halt. Shutting down the virtual machine caused everything to go back to normal, so I figured the problem was with Parallels. So I upgraded it. Same problem. I made sure Windows was up-to-date, thinking that maybe the Windows Update service was going nuts. Nothing changed. Whenever Windows was running in Parallels, the Task Manager showed the disk activity pegged at 100%.
I tried all sorts of online solutions, but none worked. On a whim, I tried it without Parallels. Windows behaved the exact same way when running natively through BootCamp. So it wasn't a Parallels problem, but a Windows problem.
Frustrated out of my mind this past week, I decided to blow away the entire partition, and install Windows Server 2016 Standard, to see if a less Desktop-oriented OS would have fewer problems. I downloaded the ISO through my MSDN Subscription and installed it to my BootCamp Partition, opting for a fresh install.
As soon as Windows and I logged in, the disk went to 100%. I felt like giving up entirely and buying a cheap laptop and simply not bothering. It was at that point, while looking for something else entirely, I hit on the solution quite by accident.
Essentially, it appears to be a driver issue with the virtual hard drive. The solution was to use the device manager to find the device identifier:
Once in there, simply look for the driver details.
This fix only applies if the driver is storahci.sys. If so, we can go back to the controller properties, and make note of the device instance path, or AHCI Controller Value.
From there, find that entry in the registry...
Since following these steps, I haven't had any disk problems. Life is good, at least where Windows on my Mac is concerned. Again, I cannot take credit for figuring this out, but I didn't want to lose it, so I'm reproducing the steps here. Thanks again for the folks at this page who did figure it out. I will be ever grateful.