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.