Under certain circumstances you may only need to mount an incomplete volume set. This may be due to a damaged member of a volume set, the need to access only certain information contained on a large volume set, or may be due to missing members of a volume set.
If you desire to mount an incomplete volume set, mount the members of that volume set you desire, and then select the Force Mount button. This will mount those members as READ-ONLY for access.
The Mount Status in the Volume Set Properties screen for that volume set with be Incompletely Mounted, with a READ-ONLY attribute.
Once the process completes the mounting of all members of the volume set you will see the volume(s) displayed under the Volume Sets view.