Stop Spotlight from indexing external hard drive

If you don’t want Spotlight to index an external driver or a USB flash drive you only have to add an empty file called “.metadata_never_index" to the root of the drive. You can do it in the terminal with the touch command. Supposing your external drive's name is "Harddrive" it would be:

touch /Volumes/Harddrive/.metadata_never_index

The same effect can be achieved in the Spotligh’s System Preferences panel using the Privacy tab. Just add, with the “+” symbol, or drag the drive (or just a folder) you don’t want to be indexed.

Ref: http://forums.macrumors.com/showthread.php?t=309194
http://osxdaily.com/2011/12/30/exclude-drives-or-folders-from-spotlight-index-mac-os-x/