Escalade ATA RAID controllers give you the option to specify a hot spare from one of your available drives. Hot spares should be selected after mirror creation. (See the section Creating a disk array in the 3ware Disk array Configuration chapter of the user's manual http://www.3ware.com/support/userdocs.asp) To specify a hot spare after the system is booted, physically add the spare drive and click on the check box next to the available port; then click on the Add Spare button.
Hot spare drives must be equal to or larger than the drives used for redundant arrays.