simple Windows dataset problem

I would like to backup only certain file types on all drives, eg. doc files on Windows Filesystems

I have tried various wildcards in Source Data Tab of Dataset, but without success:

*.doc

/**/*.doc

*:/**/*.doc

none of them are correct: "Specified source path "*.doc" does not exist; Error in path list: All paths are invalid..  Correct path list before retrying"

Users have multiple disk partitions mounted as various drive letters / paths.

What is the correct wildcard combination to backup only specific files on all connected drives?

I found workaround by using all drives as source path, exclude * and then include the specific file types, but then I cannot exclude system paths, etc.