Interesting news all around, really. Even though as it progressed it was failing to move the 100MB files, that may well be the stem of the trouble.
Sounds like the correct course of action would have been to just go home, put the new USB drive to work storing the non-working files (as in ‘not called upon,’ they work just fine), and then return another day with the FW800 lacie drive. …which is something I’ll be doing anyway.
Thanks for addressing my curiousity.