The following steps will guide you trough the mirroring of an AIX rootvg.
This info is valid for AIX 4.3.3, AIX 5.1, AIX 5.2 and AIX 5.3.

• Make sure you have an empty disk, in this example its hdisk1
• Add the disk to the vg via “extendvg rootvg hdisk1
• Mirror the vg via: “mirrorvg rootvg”
• Adapt the bootlist to add the current disk, the system will then fail to hdisk1 is hdisk0 fails during startup
  • do bootlist -o -m normal
  • this will list currently 1 disk, in this exmaple hdisk0
  • do bootlist -m normal hdisk0 hdisk1
• Run a bosboot on both new disks, this will install all software needed for boot on the disk
  • bosboot -ad hdisk0
  • bosboot -ad hdisk1