Mac storage basics · Memory, swap and speed
Does Spotlight indexing use disk space, and can I rebuild the index?
Short answer: Spotlight keeps a modest index on each indexed drive. To rebuild it, add the drive to Spotlight's Privacy list in System Settings, then remove it; adding it alone stops indexing.
Spotlight keeps a search index on each drive it indexes. It's usually modest compared with the files themselves, and indexing runs in the background after big changes, which can briefly use CPU and disk.
To rebuild a broken index, add the drive to Spotlight's Privacy list in System Settings, wait a moment, then remove it again — Spotlight reindexes from scratch. Adding a drive to Privacy also stops it being indexed at all, which some people do for backup drives.