Sometimes your Spotlight starts to behave by re-indexing your disks over and over again, and you’ll either need to clear the Spotlight index for the drive or just disable the indexing for that particular volume if you don’t use the search often. The Mac OS X Spotlight is controlled by the ‘mdutil’ tool on Snow Leopard and Lion, so first of all get the current state of the things by typing this into your Terminal session:
mdutil -sa
The output will show the Spotlight index status for all connected volumes.