Disk is newly added and you didn't perform a reconfiguration boot 

Go to Chapter 34, SPARC: Adding a Disk (Tasks) or Chapter 35, x86: Adding a Disk (Tasks).

Disk 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.

Disk is not properly connected to the system 

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