If during the mksysb restoration process you get the error Over mounting /., you most probably have some missing filesets on the spot that should be present for the mksysb you are using.

The best way is to recreate the spot from the very mksysb image you are trying to restore.

Source mksysb image:

[gzaix:root:/home/root:] lsnim -l mksysb_server1
mksysb_server1:
   class         = resources
   type          = mksysb
   Rstate        = ready for use
   prev_state    = unavailable for use
   location	 = /export/mksysb/server1.14mar2012.sysb
   version	 = 5
   release	 = 3
   mod           = 12
   oslevel_r     = 5300-12
   alloc_count   = 0
   server        = master
   creation_date = Thu Mar 15 02:06:46 2012

Create a spot from it:

Define a Resource

Type or select values in entry fields.
Press Enter AFTER making all desired changes.

                                                        [Entry Fields]
* Resource Name                                      [spot_server1]
* Resource Type                                       spot
* Server of Resource                                 [master]                                +
  Source of Install Images                           [mksysb_server1]                        +
* Location of Resource                               [/export/spot/]            /
  NFS Client Security Method                         []                                      +
  NFS Version Access                                 []                                      +
  EXPAND file systems if space needed?                yes                                    +
  Comments                                           []
 
 installp Flags
  PREVIEW only? (install operation will NOT occur)    no                                     +
  COMMIT software updates?                            no                                     +
  SAVE replaced files?                                yes                                    +
  AUTOMATICALLY install requisite software?           yes                                    +
  OVERWRITE same or newer versions?                   no                                     +
  VERIFY install and check file sizes?                no

Command: running       stdout: yes           stderr: no

Before command completion, additional instructions may appear below.


 Creating SPOT in "/export/spot/" on machine "master" from "mksysb_server1" ...



 Restoring files from BOS image.  This may take several minutes ...



 Checking filesets and network boot images for SPOT "spot_server1".
 This may take several minutes ...

Command: OK            stdout: yes           stderr: no

Before command completion, additional instructions may appear below.


 Creating SPOT in "/export/spot/" on machine "master" from "mksysb_server1" ...

 Restoring files from BOS image.  This may take several minutes ...


 Checking filesets and network boot images for SPOT "spot_server1".
 This may take several minutes ...

warning: 0042-001 m_mkspot: processing error encountered on "master":
   0042-175 c_ch_nfsexp: An unexpected result was returned by the
        "/usr/sbin/exportfs" command:
        exportfs: /export/mksysb/server1.14mar2012.sysb: No such file or directory

Of course, warnings may appear but don't worry, you should be OK from now on.