Hello cgrant, and thank you for getting back to me on this matter.

I admit I changed to code to use the default schema and didn't preserve the original code.

However, reading your answer, it's likely that the error actually originated from the readStream statement rather than the writeStream statement, in which case the cause of the error would be typing a bad Volume path in the "source" variable, calling readStream, and subsequently not re-running the cell containing readStream. I'm going to accept your answer as the (very likely) solution based on this, thank you.