I guess You are trying to restroe from a backup to disk location . The media which contain the Grt backup as to be online to expand the individual selection . For eg if you have backup on removable usb drive you can expand the mailboxes inthe restore selection when the drive is connected if the drive is not connected to the server it does not allow you to expand the Grt selections.
If the backup is on tape you can expand the selection as the catalog is written to the catalogs driectory of backup exec . But when you have Grt backup on disk the catalogs a re not written , they are generated automaticall using the IMG folder ..
Let me know if this answeres your concern