This guide assumes you have already installed the 3ware software tw_cli. Which is a command line tool to configure 3ware raid controllers. To use an emtpy unconfigured disk as a spare raid disk do the following:
tw_cli /c0 add type=spare disk=1
Creating new unit on controller /c0 ... Done. The new unit is /c0/u1.
The disk number is the port number of the disk you would like to use. To get a list of all the attached disks use the folllowing command:
tw_cli /c0 show
Please do not use any of these commands without exactly knowing what you are doing. You can lose all of you data or delete you entire raid array. Please check the man page: