Bremsstrahlung

This is so useful (not)!

Niobium-II:~ $ mdutil -s /Volumes/Heimdall - Time Machine/
/Volumes/Heimdall - Time Machine:
    Indexing enabled.
Niobium-II:~ $ mdutil -i off /Volumes/Heimdall - Time Machine/
/Volumes/Heimdall - Time Machine:
    Indexing enabled.

The two commands issued were to check the status and to turn off Spotlight indexing of the given volume, respectively. Unfortunately, the turning off step sort of does. . . nothing. A clearer message as to why it chooses to do this would really be nice. Apparently the fact that this is a Time Machine Backup volume causes this, although weirdly it seems the 'Privacy' tab in the gui Spotlight preferences can do the job, where the command line tool cannot.

Update August 13: So when you do use that privacy tab to deny indexing of entire volume, mdutil will logically enough report that indexing is disabled. . . and will be unable to re-enable it. It works just like above, except that you ask mdutil to enable indexing, and it cheerfully comes back reporting that indexing is still disabled. Again, it would be good enough to simply indicate that indexing is disabled via another mechanism which mdutil cannot override.