I created some new users who should have mailboxes, but when I look at the M pseudo drive (which the Exchange Installable File System—ExIFS—creates), the mailboxes aren't there. Why not?

Exchange 2000 Server is smart: It doesn't try to create a mailbox until it needs to deliver mail to it. You can force new users' mailboxes to appear by sending mail to them or by using Outlook Web Access (OWA) 2000 to log on to each one. When you do so, the store will create the database entities representing the mailbox, at which point they'll appear on the M drive as well.