If post-processing works manually. Then at least you know that it can determine the Showname, and move/rename to the right drive/folder.
Why it's not doing this automatically?
It can either be in how the downloaded file is handed over to the SickBeard post-processor.
Which download client do you use? and how do you send the 'result' to SickBeard? Blackhole or script. Maybe a configuration error.
Maybe your mac is not in the same state as when you do it manually.
And by this I mean maybe the external drives are sleeping or remounted with another name or something.
In both cases you can take a look in SickBeard view log> debug and try to see why it's not working.
If you use SABnzbd with sabtosickbeard script then you can also take a look in SABnzbd view script log (more )