Prevent Spotlight from indexing external HD's

“If you, like me, get annoyed by the index files Spotlight creates any time you create an external hard drive, you might want to try this hint. Were going to set up a launchd service which runs a script every time a hard drive is mounted. This script will touch the .metadata_never_index file which prevents Spotlight from creating the index; see this hint on the hard drive.”–via Mac OS X Hints.