External hard drive not indexed by Spolight on Mac OS X Mountain Lion
From the Terminal application type the following:

mdutil -s /Volumes/&lt;your_disk_name&gt;/


If indexing is disabled you will get something like that:

/Volumes/&lt;your_disk_name&gt;:
Indexing disabled. 


You can turn it on by typing (it will request your password):

sudo mdutil -i on /Volumes/&lt;your_disk_name&gt;/


Now erase all the existing entries in the index for that drive (if any):

mdutil -E /Volumes/&lt;your_disk_name&gt;/


Check that indexing has been enabled:

mdutil -s /Volumes/&lt;your_disk_name&gt;/


If indexing is enabled you will get something like that:

/Volumes/&lt;your_disk_name&gt;:
Indexing enabled. 


Now just reboot your system or wait some minutes until Finder begins the indexing.