Your email client has to know where the mail is stored, David, and if you move the storage location, then you will have to tell the email client where to look for it, and where to put new mail.
Two things: what's your email client (Outlook, Outlook Express, etc), and what's the purpose of moving the data? Is it just to back up and move to another machine, or are you intending to use the external drive permamently?