It seems that some additional configuration is required fo ACS robot.
(from Media Manager System Administrator's Guide)
http://support.veritas.com/docs/268103
Media Access Port Default for ACS Robots
MAP_ID = robot_num map_ID
This configuration entry applies only to NetBackup Enterprise Server.
This entry in vm.conf sets the Media Manager default for the Media Access Port that may be used for ejecting media from ACS (Automated Cartridge System) robots. This default is highlighted as a choice in the Media Manager and Vault administrative GUIs, but the user can also select other Media Access Ports for ejects.
If the access port specified by the MAP ID entry is not available or this entry is not present, the default media access port selection process will be used. This selection process matches the number of media specified to be ejected to the smallest access port that will hold that number of media, and uses that port.
If multiple MAPs are selected by the user, the MAP ID entry is not used and the nearest-MAP algorithm is used (see “Adjacent LSM Specification for ACS Robots” on page 383).
robot_num is the robot number. map_ID is in the format of an ACS CAP (Cartridge Access Port) ID and cannot contain any spaces.
The following example specifies the MAP ID for ACS robot number 700. The ACS CAP ID of 0,1,0 is used.
MAP_ID = 700 0,1,0
