Move Exchange Mailboxes help

I have a Windows NT 4.0 sever with Exchange 5.0 on that is running out of disk space. I am out of space to install a new hard drive so I want to know if any one has any ideas on how I can add a new drive to put the mailboxes on. I was thinking about an external SCSI hard drive connected to a SCSI card. Would this be a reliable option? Is there a maximum size of partition that Windows NT can see? Also what is the best process to move the Exchange information to a new drive?