If I add a Datafile (.NDF) on another Drive and then set the .MDF file to NOT autogrow, will the .MDF stop growing altogether and data will be written to the .NDF going forward?
Yes. If you set the file not to grow, the file won't grow. SQL will still write a little to that file until it's full but it won't grow any further.
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: Discussions on DB performance with occasional diversions into recoverability
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