The Move Disk Load dialog box has a slider bar for selecting a percentage of disk load (disk visits) to be moved, and a panel that displays a list of disks from which you can select a target disk. The disk from which load will be moved is the one you selected in the I/O window.
The Workload button in the I/O window is used to select the class for which diskload will be moved.
See Also Moving Diskload to Another Disk