How Can I Exclude the Backup Drive When Searching for a File Using the Finder?

How can I exclude the backup drive when searching for a file using the Finder?
I use TimeMachine to backup my computer to a Western Digital My Book Hard Drive.
But every time I search for a file Spotlight searches also the Western Digital My Book Hard Drive.
How can I exclude the My Book Hard Drive when searching for a file?
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Erich