Following steps need to be performed in a sequential order

1) Determine which drive is the new drive.  Use the ioscan -fnC disk command or obtain help from an HP hardware engineer (HP-CE) or the HP Response Center.  

The disk device name  for this process:
     -Example: /dev/dsk/c3t2d0 (Controller instance 3, SCSI ID 2, Lun 0)

2)  Partition the new disk and populate. Use vi to create a partition description file by doing the following:
      # vi /tmp/partitionfile
      The three line entry for HP-UX 11.22 should look like this:

         2
         EFI 100MB
         HPUX 100%

      The four line entry for HP-UX 11.23 should look like this:

         3
         EFI 500MB
         HPUX 100%
         HPSP 400MB


   3) Use idisk to set up the disk partitioning using the file created above:
      # idisk -wf /tmp/partitionfile /dev/rdsk/cXtXdX

   4) Use insf to create the new device files (cXtXdXs1, cXtXdX2 and cXtXdXs3)
      # insf -e

   5) Use mkboot to format and populate the newly created EFI partition.
Note: This command is done to the whole disk path.
      # mkboot -e -l /dev/dsk/cXtXdX

   6) Use vxdctl to have Volume Manager see the new cXtXdXs2 disk:
      # vxdctl enable

   7) Use vxdisk to confirm that the new disk can be seen by Volume Manager and that it is "online invalid":
      # vxdisk list

   8) Use vxrootmir to mirror the root disk. Make sure to use  the -v option since mirroring volumes takes a long time:
      Note: The path must be to sub-partition s2)
      # /etc/vx/bin/vxrootmir -v cXtXdXs2

    9) Use vxassist to mirror all non-root volumes on rootdg.
      # vxassist -g rootdg mirror VOL_NAME

   10) Use vxprint to verify that all volumes are mirrored:
      # vxprint -g rootdg -vp  (All volumes should have two plexes and  be ACTIVE)

   11) Make the mirror disk as alternate boot disk:

   # setboot -h

   12). Create a new EFI boot option.

    13)  Reboot and select the EFI "Boot Option Main menu" --> "Add a boot Option", then select the correct disk.  Select "EFI" --> "HPUX" --> "HPUX.efi". Then name the entry to either "HP-UX Primary boot" or "HP-UX alternate boot"

     Selecting the mirror disk. Example,
      (Pun1,Lun0) represents SCSI target 1, Lun 0 (cXt1d0):
      (Pun2,Lun0) represents SCSI target 2, Lun 0 (cXt2d0):
     ---------------------------------------------------------------
     EFI Boot Maintenance Manager ver 1.10 [14.61]

     Add a Boot Option.  Select a Volume

         IA64_EFI [Acpi(HWP0002,100)/Pci(1|0)/Scsi(Pun1,Lun0)/HD(Part1,SigB45A0000)
         IA64_EFI [Acpi(HWP0002,100)/Pci(1|1)/Scsi(Pun2,Lun0)/HD(Part1,Sig958B0000)

   14)  If required, select "Change boot order" to manage the boot order.  

For example, "HP-UX Primary boot" should be on the top, followed by the "HP-UX alternate boot" boot option.

Verify the process completed successfully by manually booting the system using each boot option

Finally, run the following command after booting HP-UX:
       # setboot           (verify that both disks are in the bootpath)
       # vxprint -g rootdg -pv (all vols should have two plexes and be ACTIVE)