Based on your comments above, can you confirm that you want me to delete the DATA (D:) volume on Disk 3, so that I have 100% completely unallocated space.... and that this action would allow for the "Add Mirror..." menu to be available on the DATA (E:) volume on Disk 1?
Sorry for being so explicit here but I just want to make sure that - under the circumstances - there is no chance for some miscommunication.