dovecot - Oct 2014 - Namespaces (or another way) to put messages in a different drive

Hello people,

I have a situation where I need to create a mailbox for clients where they put
old messages (> 1year) but I would like to create this mailbox in a different
disk.

I have a server with the mailboxes and a storage mapped using NFS just to create
those mailboxes for old messages. I was trying to namespaces but every time that
I make the configurations, the authentication system stop and the clients can?t
log in.

I really don?t know if this is the best approach for my situation (mailbox with
old emails in a storage) and, if namespaces can solve it.

The infos that I have are:

Dovecot version 2.1.7
mailboxes: 

Namespace configs:

namespace {
    type = private
    separator = /
    prefix     #location defaults to mail_location.
    inbox = yes
}

namespace {
        type = private
        separator /
        prefix = storage/
        location = /mnt/exibstor/%u
        inbox = no
        hidden = no
        list = yes
}

If someone knows how can I do that, I really appreciate it.

Best regards,
Paulino Michelazzo