The problem PC is my Dell 755, which I got refurbished on eBay .
Patio, I now have Windows 10 on a 500 GB hard drive that has passed the standard tests. The drive is partitioned so I can have more that one OS and have a partition to store my personal stuff. I also have another 500 GB drive to use as a backup.
At this point it seems to be stable. I am not sure what was the problem. I tried four different hard drives and could not isolate a problem on three of the hard drives.
The bad hard drive has been retired now. It was not the 40 GB.
I am going to clone the PC hard drive in my Dell and try the April Windows 10 update again.
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My other PC is a newer Gateway with a 64 bit version of Windows 10. I did not subject it to the April update. I am going to wait before I do anything to harm it.
The Gateway does not have any problems.
(Except the problems ones I create myself.)