Woe be unto thee, O disks!
Hah! Even though good blocks were leaping off the platters like rats from off the Titanic, I managed to outsmart the bugger. It occurred to me that I still had my old /usr partition, unused, on the original disk. I never zapped it after copying it to the larger partition on the new (now discovered to be bad) disk. So: play some cabling games to get both disks into the system, boot with the bad one, use it to copy the old partition to the new disk, shutdown, swap disks and remove the bad one, reboot, rpm --rebuilddb, up2date -p, up2date -ui, and I seem to be back in business. (Those last few commands were to handle any RPMs I had installed since the original partition move from the installation of the bad disk.)