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1 Option/Index Manager/Update Now
This means syncing files to the index.
2 Sync A Folder
This means syncing files in a spec folder to the index.
This means deleting all index data first and rebuilding the index database.
4 If you just want to only indexing a specific folder.
Go to Option/Index Manager/File type, Ctrl+A select all file types, and click edit to set your folder.