How can I mount disks via cron in Mountain Lion?
I had this same problem and found that I could get it to work if the cron job was launched under root.

This Apple discussion link has this note:


  [...] Basically, Apple is tracking where an application launches from,
  and seems to be preventing processes that launch from "cron" (one of
  the system schedulers), even indirectly, from mounting local disk
  volumes, even when that application is signed.