Reader Alan Conway has a forgetful Mac. He writes:
Lately, when I search in Mail for messages from a particular sender, only older matching messages appear. When I scan down the list of recent messages I've received I see other messages from that sender that didn't appear in the search results. What's wrong?
This is a sign that Spotlight has lost its way. When this happens you should reindex your hard drive. The easiest way to do this is to launch System Preferences and select the Spotlight preference. Click on the Privacy tab and then drag your hard drive's icon into the location area. Confirm that you want to do this by clicking OK in the resulting dialog box. Now select the hard drive you just added and click the Minus (--) button at the bottom of the location list to remove it. Spotlight will then reindex the drive.
What's happened is you've essentially told Spotlight, "Please forget about anything you've indexed on this drive." Spotlight complies by clearing out the index for that drive. When you then remove the drive from the location list, Spotlight notes that the drive has no index and proceeds to create one.