Extend a volume should have an additional warning when selecting non-matching number of disks
This is the current behavior in the legacy UI:
If an attempt is made to add a non-matching number of disks to the
existing vdev, an error message appears, indicating the number of
disks that are required. Select the correct number of disks to
The new UI does warn at the beginning that the user should use the correct number of devices but does not enforce it if they do not. It needs to enforce it for feature parity.