Privacy List Error – Spotlight

I got this error message when trying to add a folder on an external disk drive to the Spotlight Privacy area in Preferences.

“Privacy List Error

The item couldn’t be added or removed because of an unknown error.”

This forum suggests deleting this file:
/.Spotlight-V100/Store-V1/Exclusions.plist
and then restarting Finder.

http://discussions.apple.com/thread.jspa?threadID=1462997