By default, Data ONTAP does not allow you to mix disks with different RPM speeds.
Select the checkbox if you want to override this setting and allow the addition of mismatched disks selected in the Disks to Add field. If you have selected disks with different RPM speeds, you will not be allowed to add the disks unless you select the checkbox. This setting is not saved; you must select the checkbox each time you choose to override the default setting in order to add mismatched disks.