Setup mailboxes at external disk with postfix & courier

I have been investigating this for a long time but I'm not able to find a solution: I have an up&running mail server with Postfix and Courier in an Ubuntu Server. I'm using PAM users with a Maildir mailboxes structure. All this is working in the same hard disk. The problem is that I need to install/configure users mailboxes in a bigger disk unit already configured in my server ("sda" is where ALL is installed and "sdb" is where I want to setup the users mailboxes). Is not needed to migrate actual mailboxes. I,m not able to say to Courier to "look" at the users Maildir in the "sdb" drive when the mail client check the mailbox. Any help will be very appreciated. Thank you all!.