The content index has already been fed into a content collection on the FAST Search for SharePoint backend. You must clear ...

The content index has already been fed into a content collection on the FAST Search for SharePoint backend. You must clear the content from this specific content collection on the backend to ensure data remains in sync. To do this, use PowerShell commandlets. Load the Microsoft.FASTSearch.Powershell snapin and use the command Clear-FASTSearchContentCollection. Note that this is irreversible. Ensure that you clear the same collection as used by this service application.