When you set up this second server, how did you do it, were all the drives fitted before installing the software? If so, you will need to either remove a drive and then re-add it, or just add another drive. When you add it, you will get the option of whether you want it to be used as part of the storage pool, or for backups. You will need to choose backups, otherwise it won't show up in the C & B tab.
Colin