Worse than before. Took about 20 minutes to load from the drive (instead of 23 seconds...), and then failed to uncompress the kernel. Didn't even get as far as booting. A second attempt took only 5 minutes to load, but still failed on a CRC. Meanwhile, my own CD drive was non-functional.
I also tried booting and verifying with my own drives in place, but with one or 'tother unplugged. Still got the same behaviour, which is that disks 1 and 2 verify okay, and disks 3 and 4 don't. Yet they all verify perfectly on machines at work.
Sigh. I'm doomed. Doomed, I say.