A system disk contains the root (/) or /usr file systems, or both. If the disk containing either of these file systems becomes damaged, you have two ways to recover:
You can reinstall the entire Solaris environment.
Or, you can replace the system disk and restore your file systems from a backup medium.
A secondary disk doesn't contain the root (/) and /usr file systems. It usually contains space for user files. You can add a secondary disk to a system for more disk space or you can replace a damaged secondary disk. If you replace a secondary disk on a system, you can restore the old disk's data on the new disk.