This seems slightly different from the Mal problems I've seen so I thought I'd make a separate post. I've had similar problems every time they do a major overhaul of Mail and OS at the same time.
Mail tries to import the mail which is supposedly 225K e-mails. I don't believe it is that high, I believe it is 1/4-1/2 that, but some of them have gotten duplicated by Mail's bizarre system. Mail has always worked pretty well despite its weirdness and I couldn't find a decent alternative.
In any case, it get to 52K something e-mails and always stops in the same place and hangs up. If I cancel, it closes. If I let it go, it eventually gives me an error message and you have to close it. For some reason, Apple doesn't let you open the program and then LET you import your emails, it forces you to, and that makes it a whole lot more difficult to diagnose.
I've been slowing tryng some of these tricks of moving or deleting files/folders, not wanting to go too far and get in a mess, but so far, the process repeats exactly. The odd thing is that the files all appear to have been moved to V3 folder. And this is the new folder, correct?
So the question is if there are any other ideas? It just hung up at 52K yet again.