7.2.1 Including Different-sized Disk Drives in a Logical Drive
You can combine disk drives of different sizes in the same logical drive. If the logical drive includes redundancy, however, the size of each segment can be no larger than the size of the smallest disk drive. (See Selecting the Best RAID Level for more information about redundancy.)
To create a logical drive with disk drives of different sizes, follow the instructions in Creating a Logical Drive on a New Array. When the wizard displays the RAID Members panel, select different size drives, as shown in the figure below, then complete the wizard.
When the logical drive is created, check its resources on the Storage Dashboard: it should appear similar to the next figure, where a RAID 5 logical drive includes two disk drives of one size and one of another.