For reasons unclear other than the massive donations to the Ghod Murphy by buying a third hard drive and spending many hours in clone after clone, the problem with my drives not booting has not recurred. But today I found the first piece of software that is gone and for which I can no longer find the installation disk: Paint Shop Pro, which I use for basic photo editing, cropping, etc. I had Dell PSP Studio, which Corel no longer supports, so I had to buy the newest version (they did recognize me as a past customer, so I got the upgrade price rather than the new-user price). Phooey. I hope there aren't any other significant applications that I've not noticed.
A couple of days ago, I copied all of the identified installation files (not including PSP, which I just downloaded tonight) that were on the salvaged hard drives to a Data DVD. And I'm running drive clones at least daily, always cloning to the older of the two backup clones.
I ran the entire slate of Dell diagnostic tests on this machine (I found the bootable diagnostics disk) and there's no tests failing. Every since I've run those tests, I've had no more problems with a drive failing to boot. But it continues to make me nervous.