mrmike

Yes, you should program 1 mailbox to the new volume and it should automaticall start putting new mailboxes into the volume used last. YES, you need to rebalance.
You have 2 nodes because of the amount of ports you have. You need 2 separate processors to handle the traffic on that amount of ports, and so you will need , 2 different drives to support the 2 processors. The volumes and managed differently in multi node systems than single node. Call your vendor to have them go into the tools utility and "move the mailboxes" over to the other volume. This is kinda touchy, so have them do it late at night to avoid corruption. When they finish balancing the system, do a few FULL backups, and you shouyld be set.