I think I figured out what's happening, though not how it started. The new disk has a hosed file called BA6A74AF4F8A7. The old disk has three files with that name, in two different sizes. Postfix moves files around, putting them in one directory when it's trying to deliver them and in another when they're waiting some time. Moving a file to a directory that already has a file with that name apparently causes corruption.
Pierre -- gau do li'i co'e kei do
