New to LM: How do I refer to the local filesystem in an embedded PowerShell script of a DataSource?

That expanded ErrMsg about Param not found is also what you see in any datapoint column when it really SHOULD be showing NoData and correctly not reporting or triggering.

Actually I'm not sure what you mean here.

NaN and NoData are the same thing. If you want LM to report NoData in a script DataSource, you would just not provide that data by skipping that line in the output (or I guess providing a non-number would be equivalent). But I'm assuming your output does contain xxxxx.FileLoadError01=1 and LM is not finding that line for some reason.