I ran some hardware diagnostics for many hours without finding anything, but something told me to look at BIOS and drivers for my SATA RAID conroller. This is the device that controlls the 1.2TB of hard drives. I flashed the BIOS to an updated version, installed a new driver and cleared out another application that didn’t work and rebooted.
Then, I initiated a copy of 200GB or so of data [this would have caused the system to restart] and it’s all good so far. No unforseen reboots. So, it wasn’t hardware at all, but BIOS and drivers. This is good. We all love computers that don’t crash, don’t we?