ElFroggio, do you already have mirrored drives in your server? If so, then the process is easy and Fumetto's link will do the trick. If not, RequestedDeletion's procedure will do. You can run the backup using the server-manager backup function, either to an attached USB hard drive or over the network.
Note that, as I understand it, SME Server won't boot from a drive larger than 2 TB.