. . the day before.
Now just as a test I did try the older "unstable" media set I'd used the previous time (which did install on a non-SATA VIA chipset but not the SATA one) to see if there was a difference, but no - no difference.
LSI firmware flashed with latest version from last month, too - a year more recent than the flash for the previous install (which did fix one VIA problem that would affect all operating systems).