Is newly added and you didn't perform a reconfiguration boot 

Go to Chapter 30, SPARC: Adding a Disk (Tasks) or Chapter 31, IA: Adding a Disk (Tasks).

Is a third-party disk 

Go to "Creating a format.dat Entry".

Label was corrupted by a system problem, such as a power failure 

Go to "How to Label a Disk".

Is not properly connected to the system 

Connect the disk to the system using your disk hardware documentation.