Build your temp machine with exchange and add to exchange domain as second server, move over the datastores while live and active on both exchange boxes. this will ensure consistency on the mailboxes, remove the old server through the exchange console and bring it down. rebuild your old box and reverse.
simple solution and easy to accomplish
hope this helps
ever considered the solution of ISCSI ?
it will allow you to keep your original server online and put additional hd’s in thought the network.
you can use a NAS for that purpose.
good luck friend.