When migrating from Exchange 2003 to Exchange 2007, I had problems with disk space on one of our network drives. For the life of me, I couldn’t figure out what the problem was until we had about 100MB left. Then I realized that every single mailbox move was being logged! Try running a script that moves hundreds over at a time and then think of the log files!
I ran across a blog post that details just this and what to do about it. Hopefully you won’t end up like me finding out after the fact!